Hatch timing and maternal bill colouration are associated with chick growth in a mutually ornamented seabird

In species with obligate bi-parental care, investment by both parents in a current reproductive bout is critical to offspring growth and survival. The degree to which an individual can invest in their offspring relies on their quality as a parent. Parental quality can be communicated between individuals in a mated pair via ornamental features, as they may honestly reflect aspects of direct or indirect offspring contribution. In this study, we investigated whether the red-orange bill colouration in Atlantic puffins Fratercula arctica reflects two proxies of parental quality: hatch date and offspring growth. No aspect of paternal colouration predicted hatch date, but several metrics of maternal colouration predicted offspring peak mass and normalized wing growth. We also explored whether hatch date influenced patterns of chick growth and found that timing (early hatch vs. late hatch) but not synchrony with food availability significantly predicted mass and skeletal growth. Specifically, early hatching chicks achieved higher peak masses but exhibited reduced wing growth, potentially reflecting alternative strategies between investing primarily in weight gain or structural development. Together, these results highlight chick growth as a complex metric of parental quality, associated with both phenology and parental phenotype. Hatch timing and maternal bill colouration are associated with chick growth in a mutually ornamented seabird

In species with obligate bi-parental care, investment by both parents in a current reproductive bout is critical to offspring growth and survival. The degree to which an individual can invest in their offspring relies on their quality as a parent. Parental quality can be communicated between individuals in a mated pair via ornamental features, as they may honestly reflect aspects of direct or indirect offspring contribution. In this study, we investigated whether the red-orange bill colouration in Atlantic puffins Fratercula arctica reflects two proxies of parental quality: hatch date and offspring growth. No aspect of paternal colouration predicted hatch date, but several metrics of maternal colouration predicted offspring peak mass and normalized wing growth. We also explored whether hatch date influenced patterns of chick growth and found that timing (early hatch vs. late hatch) but not synchrony with food availability significantly predicted mass and skeletal growth. Specifically, early hatching chicks achieved higher peak masses but exhibited reduced wing growth, potentially reflecting alternative strategies between investing primarily in weight gain or structural development. Together, these results highlight chick growth as a complex metric of parental quality, associated with both phenology and parental phenotype. Key words : mutual ornamentation, parental quality, carotenoid colouration, sexual selection, puffins

One of the primary functions of animal signals is to influence the behaviour of a receiver based on the attributes of the sender (Searcy and Nowicki 2005, Owren et al. 2010). Such attributes may include the sender’s identity, current condition or breeding status, resource holding potential, or intentions or future actions (Laidre and Johnstone 2013). The content and reliability of a signal depend on the identity of the sender and the intended receiver (Laidre and Johnstone 2013). In species that form long-term monogamous pair bonds and exhibit bi-parental care, such as seabirds, some of the most critical communication occurs between two breeding adults of a mated pair. Reproduction is often a costly activity, with trade-offs between the current reproductive effort and future survival and reproductive attempts (Trivers 1972). Breeding success rests on the ability of a mated pair to communicate their current abilities and make informed reproductive decisions. While females ultimately decide if and when to reproduce (except in sexual coercion, e.g., McKinney & Evarts, 1998), other decisions require communication between both partners, leading to improved coordination of antipredator defense, incubation, and offspring provisioning (Griffith 2019). Within-pair coordination can also boost reproductive success, as demonstrated by the ‘mate familiarity effect,’ where pairs that have been together longer enjoy higher breeding performance (Rowley 1983). Many types of signals can be used to coordinate pair behaviour during a breeding attempt. Courtship behaviours such as ritual greeting displays (e.g., Nelson & Baird, 2002), elaborate dances (e.g., Nuechterlein and Storer 1982), or allopreening (e.g., Warham 1996) can ensure the pair is compatible and/or aid in partner recognition (Williams 2021). Incubation duties may be negotiated between mated pairs via acoustic signals (e.g., Boucaud et al. 2016), or a combination of acoustic and visual cues (e.g., Sládeček et al. 2019). To synchronize provisioning visits, mated pairs may assess partner vocalizations (e.g., Ferree et al. 2021), partner displays (e.g., Nelson 1978), indirect offspring cues (e.g., Granadeiro et al. 2000, Quillfeldt and Masello 2004), or colourful ornaments (e.g., Dakin et al. 2016; Limbourg et al. 2004). Of the diverse array of colourful ornaments, carotenoid-pigmented features can be especially useful in breeding contexts because they often convey information about an individual’s current condition or ability (Lozano 1994, McGraw 2006). Because parental care is an energetically costly activity that can have direct effects on body condition, only individuals that can offset this cost are predicted to invest in the current reproductive attempt (Kokko 1998, Houston et al. 2005). Therefore, a signal that transmits information on condition or ability may serve as an indicator of an individual’s parental quality. For receivers, signals encoding parental quality information may be pivotal in deciding whether to continue investing in a reproductive attempt. Ultimately, parental quality is a key determinant of avian reproductive success and thus fitness (i.e., Arnold et al. 2004; Coulson and Porter 1985; Ens et al. 1992). Parental quality is typically characterized by the degree of offspring investment (Trivers 1972). In birds, investment may be quantified prior to hatching via egg size or content (e.g., levels of carotenoids or immunoglobins; Blount et al. 2002), as well as the frequency and duration of incubation visits. After hatching, investment can be determined from parental provisioning effort, calculated as the rate of feeding events or meal sizes (i.e., Gladbach et al. 2009). Parental provisioning rate can vary over the breeding season, such that paired individuals may adjust provisioning rate based on changes in their condition and ability, signals from chicks indicating nutritional need, or signals conveying their partner’s condition and ability (Rector et al. 2014, Gillies et al. 2022). In tree swallows Tachycineta bicolor and blue tits Cyanistes caeruleus, plumage colouration honestly signals aspects of attractiveness, reproductive performance, and survival, and can be used to make decisions about offspring investment. For instance, male and female tree swallows exhibit negative differential allocation/reproductive compensation by increasing their provisioning rate when paired to a partner with undesirable plumage colouration, effectively compensating for the inability of their low-quality partner to provide parental care (Gowaty et al. 2007, Dakin et al. 2016, Haaland et al. 2017). In contrast to tree swallows, female blue tits exhibit positive differential allocation by decreasing provisioning rate when paired to a male with experimentally diminished ornamental colouration, thereby reducing their investment when paired to a lower-quality partner and limiting unnecessary tolls on survival and reproductive success (Burley 1986, Limbourg et al. 2004, Haaland et al. 2017). In species where provisioning rate might be challenging to measure, such as seabirds, parental quality can be assessed based on offspring outcomes. For example, the yellow eye and head plumage of yellow-eyed penguins Megadyptes antipodes was found to predict mean annual breeding success (Massaro et al. 2003), and the red-yellow integument colouration of male black-legged kittiwakes Rissa tridactyla has been correlated with offspring fledging success (Leclaire et al. 2019). The Atlantic puffin is an ideal candidate to further explore signals of parental investment in seabirds, as both males and females display a conspicuously red-orange bill and bright orange rosette during the breeding season and exhibit high degrees of parental care (Harris and Wanless 2011). The adaptive significance of the bill and rosette is currently unknown, but as a dynamic carotenoid-pigmented feature (Doutrelant et al. 2013, Kochvar et al. 2024) it has the potential to honestly signal individual quality. While the relationship between puffin bill colouration and current body condition is unclear (Doutrelant et al. 2013, Kochvar et al. 2024), relationships to other aspects of quality, such as foraging ability or overall health status, have yet to be investigated. It is therefore possible that bill colour could be linked to an individual’s ability to successfully raise offspring. We chose to examine hatch date and chick growth as proxies of parental quality. In many avian species, timing of egg laying (and thus hatch date) is an important factor in determining breeding success, with those that lay earlier or more synchronously with food availability being of higher quality and producing nestlings in better condition (e.g., Burger et al. 1996; Moreno et al. 1997; Gaston et al. 2009). In puffins, fledging success is higher for early hatching chicks, providing support for early hatch date as an indicator of high parental quality (Nettleship 1972, Harris 1980). Synchrony with food availability may be less salient for puffins, as breeding success remained unchanged in years where hatch date and capelin spawning were remarkably asynchronous (Regehr and Rodway 1999). However, this “mismatch” hypothesis warrants further attention, as there is evidence that synchrony with food availability is key to growth and survival in other seabirds (Stenseth and Mysterud 2002, Watanuki et al. 2009, McKinnon et al. 2012), and capelin spawning may be particularly variable in warming ocean climates (Buren et al. 2014). Chick growth, while not a direct measure of parental effort, is molded in large part by the indirect benefits parents provide. As such, chick growth often closely reflects parental provisioning effort (Martin 1987, Sæther 1994). In this study, we explore whether puffin bill, cere, and rosette colour are predictors of hatch timing or offspring growth, both of which influence reproductive success and can be used as proxies of parental quality. Since the bill and other fleshy integuments are known to be carotenoid-pigmented features in puffins (Doutrelant et al. 2013), we also ask whether any potential links between colour and parental quality could be mediated by adult body condition. Study site and species This study was conducted on Gull Island in the Witless Bay Ecological Reserve of Newfoundland and Labrador, Canada (47.26, -52.77). The Atlantic puffin colony on Gull Island is one of the largest in the Northwestern Atlantic, with approximately 120,000 breeding pairs according to a 2012 population survey (Wilhelm 2017). Puffins are long-lived (maximum known age in the wild: 33 years; USGS 2023), socially and genetically monogamous seabirds with high interannual survival and low divorce rates (Harris and Wanless 2011 and references therein). Breeding in this species is characterized by obligate bi-parental care of a single chick, with equal levels of care provided by males and females (although roles differ slightly; Creelman and Storey 1991). Pufflings (i.e., puffin chicks) are fully reliant on parental foraging trips to survive and properly develop, and parents can adjust their provisioning rate as a function of offspring need communicated via begging calls (Harris 1983, Johnsen et al. 1994, Cook and Hamer 1997, Dahl et al. 2005, Rector et al. 2014, Fitzsimmons 2018). Field methods Hatch date To assess the relationship between bill colouration and parental quality, we monitored and targeted Atlantic puffin burrows for adult capture during the 2022 breeding season. The key details of our burrow monitoring procedure are laid out in Kochvar et. al., (2024). In brief, we first identified occupied burrows using a burrowscope camera (EMS2021 Gopher Tortoise Camera System with infrared detection, Environmental Management Services, Canton, Georgia, USA) and inspected every 3-5 days for evidence of hatching. Hatch date was determined based on the contents of the burrow at each visit. Since exact hatch date could not be determined and was especially crude for the earliest hatch dates (7 chicks hatched between visits on 13 June and 24 June; logistical difficulties precluded more frequent visitation), nestlings were categorized based on timing within the breeding season. Each chick was classified as either an early hatcher (18 June – 29 June) or a late hatcher (30 June – 18 July), using 29/30 June as the cut-off date because 29 June was the median hatch date and 30 June was the mean hatch date among the monitored burrows. Coincidentally, the median hatch date (29 June) was also the day when the most important nestling food resource, capelin, was confirmed to be spawning in Witless Bay (eCapelin 2017). Considering the median hatch date as the beginning of the period of peak food availability (Regular, 2014), chicks could also be classified as either synchronous (i.e., within ± 3 days from capelin spawning; 26 June – 2 July) or asynchronous (i.e., outside of this range; 18 June – 25 June, 3 July – 8 July). Of the 39 total chicks, 25 were classified as early, 14 as late, 16 as synchronous, and 23 as asynchronous. Chick data collection As outlined in Kochvar et al. (2025), we measured each chick three times during the linear growth period (10-, 20-, and 30-days post-hatch) and every three to six days thereafter until we recorded them at fledging size (wing length >= 130 mm) for a total of at least five visits per individual chick. We measured mass with a 600 g Pesola to the nearest 5 g, flattened wing chord length with a stopped ruler to the nearest 1 mm, and tenth primary feather length (hereafter p10) with a stopped ruler to the nearest 1 mm once they had pin feathers. We banded chicks with wing chords > 125 mm using a Canadian Wildlife Service (CWS) stainless steel band. From the 49 burrows with a confirmed hatchling, we collected full data (i.e., ≥ 5 visits) on 15 individuals and partial data on 8 individuals. The remainder were either unable to be measured (n = 22; typically parents dug deeper following adult capture), connected to multiple monitored burrows (n = 2), or found dead (n = 2). Adult data collection Once a chick was confirmed to have hatched, both parents were targeted for capture. We either extracted adult puffins from their burrows after nightfall (22:00-3:00), or after several failed attempts, targeted them for capture with a noose carpet. Noose carpets were fixed to the ground of the burrow entrance with gardening stakes, covered with soil, and placed so that the large loops of the nooses were protruding up from the carpet. We set up a hunting blind at the bottom of the slope with clear lines of sight to all target burrows, and at least one observer watched the slope while the traps were deployed for signs of capture (i.e., minor struggle). In all cases, birds were caught in the noose trap while exiting the burrow, so it was highly likely that the captured individual corresponded to the target burrow. Once a bird was caught, we quickly attended to the trap and carefully extracted the bird from the noose(s), a process that never lasted more than 2-3 minutes. If the bird had already been captured (identified via CWS number), we removed the noose carpet and immediately returned the bird to the burrow; otherwise, we secured them in a cloth bag and transported them to the blind to be measured and photographed. Adult data collection proceeded following the methods outlined in Kochvar et. al. (2024). In short, we first gave individuals a CWS band for subsequent identification, measured mass to the nearest 5 g and flattened wing chord to the nearest 1 mm, and took a blood sample for sex determination. We then took them into a blind, where ultraviolet (UV, 320-380 nm) and visual spectrum (400-680 nm) RAW 20-megapixel images were taken with a full spectrum converted Samsung NX1000 (following instructions from Troscianko 2018) using two 2-inch Baader lens filters. The photos were illuminated with a full-spectrum ballast that passed through a light diffuser, and all photos included white (99% reflectance) and dark (10% reflectance) optic grade standards. We cleaned the bill of debris with a toothbrush prior to photo capture and held the bill in place with a wooden bill stabilizer (see Figure 1 in Kochvar et. al. 2024). If noose carpets were deployed during these procedures, a research assistant regularly monitored the slope in case another bird was caught in a noose carpet. At no point did a bird get caught in a noose carpet while another bird was being processed. We always removed noose carpets from the entrance prior to returning processed birds to their burrows. We successfully captured and measured thirteen mated pairs (26 individuals), along with 13 additional individuals (n = 39). Full chick growth data was recorded for ten of the mated pairs and three individuals (n = 23), partial chick growth data was available for two mated pairs and five individuals (n = 9), and only hatch data was available for five individuals. Assessment of colour Multispectral images, cone catch models, and colour coordinates from regions of interest were generated following the methodology outlined in Kochvar et. al. (2024). In brief, we aligned one photo from the visual spectrum and one photo from the UV spectrum along the bill and merged the two photos in ImageJ (Schneider et al. 2012). We then converted the multispectral image to a cone catch model using the spectral sensitivity of a violet-sensitive avian visual system (λ = 410, 450, 505, 565 nm; Ödeen et al. 2010) with a standard illuminant D65 (CIE). We extracted quantum catch values from five regions of interest (ROIs): one on the tip of the upper mandible, one on the tip of the lower mandible, one on the base of the mandible, one on the cere, and one on the rosette. Finally, we modelled the quantum catch values in tetrachromatic colour space with the R package pavo (Maia et al. 2013, 2019). We extracted three key colour variables (hue VIS, hue UV, achieved saturation) from colour vectors defined by the positions of the ROI colour coordinates in tetrahedral colour space with respect to the achromatic center. Hue represents the direction of the colour vector, in terms of azimuth (VIS) and elevation (UV). Hue VIS ranges from -π to +π, such that perceived reds and purples are negative values, perceived yellows and oranges are close to zero, and perceived greens and blues are positive values (Stoddard and Prum 2008, Dakin and Montgomerie 2013) Hue UV ranges from -π/2 to +π/2, with more UV rich colours having more positive values (Stoddard and Prum 2008, Dakin and Montgomerie 2013). Because hue VIS ranged from red-yellow and UV reflection was negative across all patches (Kochvar et al., 2024), we did not have to account for overlap at the -π to +π or -π/2 to +π/2 boundaries. Chroma is the saturation of a colour and is defined as the magnitude of the colour vector ( r ) from the achromatic center (Stoddard and Prum 2008). Achieved saturation is simply the magnitude controlling for the potential maximum chroma of the given hue ( r/r max ; Stoddard & Prum, 2008). We calculated these measures for all four chromatic regions (upper and lower mandible, cere, and rosette). Brightness was our single achromatic measure, which we calculated from the relative stimulation of the double cone. We measured brightness for all five regions, including the achromatic mandible base. All colour variables were calculated with the R package pavo (Maia et al. 2013, 2019). Molecular methods We determined the sex of the 39 adults molecularly from blood samples collected in the field following the methods outlined in (Kochvar et al. 2024). Briefly, we extracted DNA using a DNeasy® Blood & Tissue Kit (Qiagen Inc., Toronto, ON, CA) following protocols outlined in the DNeasy® Blood & Tissue Handbook (2020) and stored the extractions at -20 °C. We ran Polymerase chain reaction (PCR) on extracted DNA with an Eppendorf Mastercycler® ep gradient S to amplify the chromo-helicase DNA 1 (CHD1) gene on the avian W and Z chromosomes. The PCR was run on a program of 95 °C for 5 minutes, 94 °C for 30 seconds (35 cycles of denaturing, annealing, and extension), 50 °C for 30 seconds, 72 °C for 60 seconds, 72 °C for seven minutes (extension) and 4 °C for 10 minutes. We then ran PCR samples on a RedSafe agarose gel with 100 base pair reference ladders on a Thermo Scientific EC 300 XL for 50 minutes at 130 amps. Finally, we imaged the gels using Image Lab software and stored them digitally. Of the 39 sexed individuals, 19 were sexed as male and 20 were sexed as female. All procedures were carried out at Memorial University of Newfoundland following standard lab safety protocols. Calculation of body condition index We determined body condition from the residuals of a best fit linear regression with mass as the response variable. This was calculated using the full dataset on hatch date (n = 39) for use in all linear regression models. Typically, body condition is either given as the residuals of the linear regression of mass on wing chord length, or simply mass (Labocha and Hayes 2012, Doutrelant et al. 2013, Fitzsimmons 2018). However, mass in puffins is known to change across the breeding season and differ between the sexes, so these variables (capture date and sex) were included in our regression model. To account for potential sex-biased differences in mass change within years, we also included the two-way interaction between sex and capture date in the full model. We reduced the full model by stepwise removal of non-significant terms based on results from ANOVA tables. The final linear regression model met the assumptions of homogeneity of variance, normal distribution of the residuals, and low multicollinearity. Statistical analyses We explored whether bill colouration and parental body condition are associated with parental quality, as measured by chick hatch group and growth outcomes. Because we did not find any significant predictors of hatch group (Supplementary S1), we only report the methods and results of our chick growth models here. Predictors of chick growth We assessed whether parental bill colouration, parental body condition, and hatch group explained chick growth outcomes using linear regressions. Chick growth parameters were extracted from nonlinear growth models (mass = quadratic, wing length = logistic, p10 length = extreme value function [EVF]) as described in Kochvar et al. (2025). To avoid overstating our conclusions, we first tested whether the response variables for each growth metric were correlated using a Pearson’s correlation test. For wing and p10 length growth, all four measures (growth rate constant, normalized growth rate constant, y value at the inflection point, maximum value) were correlated (Supplementary S2, Table S1), so we only generated models for the normalized growth rate constant (r/K) because it captures both growth rate and maximum value. Since the normalized growth rate constant of wing length and p10 length were also highly correlated (0.731, p =< 0.001), and the resultant models were qualitatively the same, we only report the results of wing growth models here. Rate of mass gain and maximum mass were also correlated (Table S1), but we chose to include generated models for both response variables because: 1) normalized growth rate constants cannot be calculated for quadratic models, 2) linear growth and maximum mass are commonly assessed metrics in the literature and 3) each variable may indicate biologically different aspects of development. To assess the influence of each parent’s colouration on chick growth while avoiding singularity errors associated with overfitting, we separated the models by adult sex and colour attribute (chromatic vs. achromatic), yielding four models for each growth metric. We used Pearson’s correlations to select a maximum of five colour variables for inclusion in the full models. Since we generated models for chromatic and achromatic colour variables separately, we only calculated correlations within chromatic or achromatic variables. To reduce colour variables that were correlated (p < 0.05) within a region, we 1) retained the variable that was correlated to the most other variables (i.e., if achieved saturation was correlated to both hue VIS and hue UV, only achieved saturation was retained), and for chromatic variables, 2) prioritized hue VIS and achieved saturation over hue UV because UV reflection was low across our study sample (mean = -0.704, range = -0.375, -1.170). For the remaining colour variables, those that were correlated between regions were selected for inclusion in the full models based on the same criteria. If more than the allotted colour variables remained, variables from the same region were preferentially eliminated. Table 1 details which colour variables we retained for each full model. We reduced each model using stepwise selection from a model including all main effect colour variables presented in Table 1, hatch timing and synchrony, parental condition, and parental capture date as a control (Equations 9-20; Supplementary S2, Table S2). Parental capture date accounts for changes in bill, cere, and rosette colour across the breeding season, which are known to occur in Atlantic puffins (Kochvar et al. 2024). We chose not to include interactions between predictor variables in the full models to avoid overfitting (Babyak, 2004). To obtain our final model, we removed non-significant terms based on the P values reported in the ‘summary’ function until only significant terms and parental capture date remained. Some of our full chromatic models had evidence of multicollinearity (VIF > 5), but after stepwise removal of 1-3 terms, the remaining variables had low multicollinearity (VIF < 5). We confirmed that the final models met all assumptions of linear regressions (i.e., normal distribution and homoscedasticity of the residuals) by visually assessing R diagnostic plots. To reduce potential instances of type 1 error, we adjusted the P values of predictor variables (excluding parental capture date) using the false discovery rate correction (Benjamini and Hochberg 1995). Table 1 Colour variables included in full models for generalized linear regressions (hatch timing and synchrony) and linear regressions (chick growth) | Male | chromatic | Upper mandible hue VIS, cere hue VIS, rosette achieved saturation, rosette hue VIS | | achromatic | Base of mandible, cere, and rosette brightness | | | Female | chromatic | Upper mandible achieved saturation, upper mandible hue VIS, cere hue VIS, rosette achieved saturation, rosette hue VIS | | achromatic | Cere and rosette brightness |

Body condition index Our metric of body condition was the residuals from a regression of mass on all three main-effect variables: wing length, capture date, and sex (Supplementary S2, Table S3). Mass was positively related to wing length (3.58 ± 1.05, F 1,35 = 28.42, P < 0.001), but negatively related to capture date (-0.50 ± 0.42, F 1,35 = 4.74, P = 0.036). Mass was also higher in males compared to females (26.27 ± 8.96, F 1,35 = 8.60, P = 0.006). Factors associated with chick growth Mass Mass growth and maximum mass could be calculated for 18 pufflings, 14 of which had at least one captured female parent and/or at least one captured male parent. Therefore, both paternal and maternal mass models had sample sizes of 14 individuals. The maternal dataset originally contained 15 observations due to the presence of two females captured at a burrow. One of these females was caught in the burrow, while the other was captured by noose trap. We removed the individual caught by noose trap to avoid pseudoreplication and reduce uncertainty in paternity. The results were qualitatively the same with the inclusion of either female. The reported final models all met the assumptions of normality and homoscedasticity of the residuals. Chick mass growth was not significantly predicted by any male or female parental characteristic. However, several variables were significant in the maternal chromatic model for chick maximum mass: achieved saturation of the upper mandible, hue VIS of the cere and rosette, and adult condition were all significant predictors (Table 2). Specifically, offspring of mothers in better condition, as well as those with a yellower (compared to orange) cere, a more orange (compared to yellow) rosette, and a more saturated bill had higher maximum masses (Figure 1). Timing of hatch was the only factor that was retained in all four maximum mass models (male/female and chromatic/achromatic), with early hatching chicks achieving higher masses compared to late hatching chicks (Figure 2A). Wing length growth Wing growth metrics could be calculated for 17 chicks, 14 of which had at least one captured male parent and 13 of which had at least one captured female parent. Therefore, male wing growth models had a sample size of 14, whereas female wing growth models had a sample size of 13 chicks. After the removal of an outlier that exhibited high leverage (Cook’s Distance = 0.46, compared to next highest value of 0.10) in the male dataset, all final models met the assumptions of normality and homogeneity of the residuals. Rosette hue VIS in the maternal chromatic model was the only significant parental characteristic. Here, the opposite trend compared to mass was observed; offspring of mothers with a yellower rosette had higher wing length growth rates. Hatch timing was the only significant predictor of normalized wing growth rate in all four models (Table 2; Figure 2B). In contrast to maximum mass, late hatchers had higher normalized wing growth rates than early hatchers. Table 2 Final models predicting chick growth metrics from parental colour, condition, and hatch timing. | Maximum mass | M | C/A | Hatch timing | -60.66 ± 27.53 | -2.204 | 0.068(*) | | F | C | Upper mandible achieved saturation | 104.24 ± 35.42 | 2.943 | 0.033* | | | Cere hue VIS | 390.91 ± 87.43 | 4.471 | 0.008** | ||| | Rosette hue VIS | -159.13 ± 47.64 | -3.340 | 0.023* | ||| | Condition | 1.26 ± 0.23 | 5.378 | 0.004** | ||| | Hatch timing | -75.81 ± 10.08 | -7.518 | 0.001** | ||| | A | Hatch timing | -66.32 ± 20.74 | -3.197 | 0.018* | || | Wing growth | M | C/A | Hatch timing | <0.001 ± <0.001 | 6.318 | 0.001** | | F | C | Rosette hue VIS | <0.001 ± <0.001 | 2.851 | 0.032* | | | Hatch timing | <0.001 ± <0.001 | 4.332 | 0.006** | ||| | A | Hatch timing | <0.001 ± <0.001 | 3.459 | 0.015* | M = males, F = females; A = achromatic, C = chromatic. Only models with significant ( α < 0.05) or near significant ( α < 0.1) predictors are presented. P values were corrected with the false discovery rate and are significant at α < 0.05*, < 0.005**. (*) indicates significance or near significance before applying the false discovery rate correction. Figure 1 Several maternal features significantly predicted chick maximum mass. Two examples are illustrated in this figure: A) maternal rosette hue VIS predicted maximum mass, such that mothers with more orange rosettes had chicks with higher peak weights, and B) maternal body condition predicted maximum mass, such that mothers in better condition had chicks with higher peak weights. Figure 2 Timing of hatch significantly influenced multiple aspects of chick growth. A) Early hatchers achieved higher maximum masses compared to late hatchers, and B) late hatchers had higher normalized wing growth rate constants compared to early hatchers.

The primary aim of this study was to assess whether Atlantic puffin bill colouration reflects parental quality, as assessed by offspring hatch date, mass gain and structural growth. Chick growth was explained by some aspects of maternal bill, cere, and rosette colouration, but no aspects of paternal colouration. Hatch timing was not explained by any aspect of parental colouration, but hatch timing itself predicted several measures of chick growth. Indeed, hatch timing seemed to be one of the key factors influencing puffling growth patterns in our study (in accordance with Nettleship 1972). As we expected, parental colouration, as determined by the hue, saturation, and brightness of several ornamental features, significantly predicted peak mass and wing growth in pufflings. In most cases when colour was retained in the final model, the relationship with chick growth was in the predicted direction; redder, more saturated colours were associated with better chick outcomes. This aligns with hypotheses linking carotenoid colouration and individual quality, where higher quality individuals can invest or express more carotenoids in ornamental features and produce more saturated, redder-orange ornaments as a result (e.g., red crossbills Loxia curvirostra, Cantarero et al. 2020; red grouse Lagopus lagopus scoticus, Mougeot et al. 2007; great black-backed gulls Larus marinus, Kristiansen et al. 2006). Indeed, higher quality mothers (i.e., those that could raise a heavier chick) were also those that produced features with more carotenoid pigmentation. We observed no cases of paternal features predicting offspring growth, perhaps indicating that maternal bill colour more honestly reflects ability to provide parental care. This is puzzling considering that puffins exhibit obligate bi-parental care, with females and males providing near-equal contributions across the breeding season. Both parents are critical to successfully raising a chick; those that are raised by a single parent either die or develop much more slowly (Harris 1978). However, there are a few important differences in investment between the sexes. Females establish the initial investment in the breeding attempt, as they are responsible for egg production and accordingly the transfer of important nutrients and metabolites to the developing embryo (i.e., yolk composition; Price 1998). This aspect of investment is especially intriguing considering the demonstrated link between metrics of egg quality, such as size or hormonal content, and enhanced chick growth in some avian species (e.g., androgens in black-headed gulls Larus ridibundus, Eising et al. 2001; size in thick-billed murres Uria lomvia, Hipfner 2000; egg size across avian taxa, Krist 2011; testosterone in European starlings Sturnus vulgaris, Pilz et al. 2004). There is even some evidence that the transmission of maternal carotenoids may be linked to offspring growth, as in yellow-legged gulls ( Larus michahellis, Saino et al. 2008), but the benefits may vary according to offspring sex and timing of hatch (Romano et al. 2008) and more often extend to offspring immune function rather than growth (e.g., blue tits, Biard et al. 2007; barn swallows Hirundo rustica, Saino et al. 2003). Hormonal cycles of sex steroids in females may also affect both reproductive investment and carotenoid colouration, as high oestradiol levels in the early breeding season are critical to successful yolk and egg formation, while low oestradiol levels in the late breeding season permit the production of redder, more saturated integuments (Romero-Diaz et al. 2022). Females also contribute more to direct care of the young, even though males spend more time on the colony and are more involved in nest maintenance and defense (Creelman and Storey 1991, but see Table 3.3 in Harris and Wanless 2011 for exceptions). Specifically, females incubate eggs longer, provision chicks more frequently, and provide more high-quality nutritional items than males (Creelman and Storey 1991, Fitzsimmons 2018). Unlike males, females dynamically shift their feeding rate depending on foraging conditions, potentially indicating that females have a more flexible pattern of provisioning than males (Fitzsimmons 2018). Taken together, females seem to have more opportunities to adjust their investment across the breeding season than males. While we were unable to explore whether females and males differentially invest, it is interesting to consider how previously reported patterns of sexually dimorphic investment may influence signaling behaviour (Creelman and Storey 1991). In the context of differential allocation, it would only be informative for females to signal parental investment, since their ability to provision dynamically changes while that of males remains stable. Yet, by the same logic, males would not be expected to respond to signals indicating changes in maternal investment, since they provision steadily throughout the chick-rearing season. However, perhaps males would respond if female ornamental colour drastically changed in a way that reflected a marked decrease in parental quality. Future studies should investigate whether female bill colour has the potential to act as a signal of parental quality between mated pairs by experimentally manipulating female bill colouration during the breeding season. In puffins, the direction of the response may depend on when the decrease in partner quality occurs, and thus how much investment has already been made in the current offspring (Harris and Uller 2009, Ratikainen and Kokko 2010). If maternal investment in egg composition is minimal, and this is reflected in her bill colouration, then a male partner might be predicted to incubate and provision less or abandon the breeding attempt altogether (i.e., positive differential allocation). In contrast, declines in maternal quality, and thus ability to provision, observed close to fledging may result in a brief spike in paternal investment (i.e., negative differential allocation). While such temporal changes in investment strategy have not been documented in species with a single offspring, several studies demonstrate fluctuations in parental investment across multiple broods (e.g., Grüebler 2007, Robinson et al. 2010). Since our measure of parental quality did not consist of a direct assessment of investment (e.g., egg composition, provisioning rate), we cannot exclude the possibility that colouration reflects another aspect of individual quality. Foraging ability is one measure of quality that could potentially explain the link between maternal bill colouration and enhanced chick mass gain (per the good parent hypothesis, Hoelzer 1989; as in blue tits, Senar et al. 2002, García-Navas et al. 2012). However, this would require that 1) diet influences bill colour expression in females, and/or 2) paternal provisioning has a weak influence on offspring growth compared to maternal provisioning. While food availability undoubtedly influences avian offspring growth, genetics are also likely to affect a chick’s metabolism and ability to gain weight. This idea has received attention in the commercial poultry industry, where genetic selection for enhanced growth and development has been successful (Emmerson 1997, Buzała et al. 2015). It is therefore plausible that maternal colouration simply reflects genetic quality, and the association between colour and offspring health is a result of the direct benefits she provided. Another possibility is that females with redder, more saturated features are paired to males of higher parental or genetic quality. While annual survival is high and divorce rate is low in this species, significant mate changeover can still occur (22% of cases where both birds were banded, 9.3% of cases where both birds were present the next year, Creelman & Storey, 1991). Female bill colouration could be used by males to decide whether to stay with their mate or search for a new mate prior to breeding. Males typically solicit females in this species and may attempt to copulate with multiple females, but unreceptive females can evade or repel copulation attempts (i.e., no forced extrapair copulation; Creelman & Storey, 1991). It is difficult to ascertain which sex is “choosier” in this scenario, and in mutually ornamented species with bi-parental care, both sexes likely exhibit some degree of choosiness (Burley 1977, Amundsen 2000). Hence, it is possible that male bill colouration reflects an aspect of quality that we did not measure, and that bill colouration is important in mate choice for both sexes. While parental colouration certainly played a role in predicting chick growth, hatch date proved to be the most consistent explanatory variable. Specifically, hatch timing was significant in nearly every model, but hatch synchrony was never retained. This indicates that there is more of a distinction between early hatching and late hatching chicks compared to synchronously and asynchronously hatching chicks. However, it is also possible that our measure of synchrony did not accurately reflect match/mismatch with capelin availability, as peak capelin abundance in nearshore waters may occur after spawning is first recorded (Nakashima 1996). Nevertheless, timing of hatch was clearly an important factor in determining chick growth. Early hatching pufflings generally have higher fledging success and may therefore be expected to have higher growth rates (Nettleship 1972, Harris 1980), but this did not hold across all growth metrics in our study. Individuals that hatched early achieved higher peak masses, but also had lower normalized wing growth. This may reflect a trade-off between mass gain and structural growth, with early hatchers allocating more energy to mass gain and late hatchers investing more in wing and feather growth. Indeed, food-stressed pufflings prioritize growth of skeletal structures like the head and wings over body mass gain (Øyan and Anker-Nilssen 1996). One possibility is that late hatchers invest in rapid structural growth so they can successfully fledge and hunt on their own at a younger age. The length of the chick rearing period is flexible in puffins, but there is likely an upper limit to this flexibility, as parents tend to reduce provisioning and/or abandon the attempt after the young have reached a certain age (Johnsen et al. 1994, Erikstad et al. 1997). Puffin parents may be even less flexible toward the end of the breeding season, since both successful and unsuccessful breeders tend to leave the colony around the same time, if not the same day (Harris & Wanless, 2011). Therefore, late hatching chicks may not be able to prolong their rearing period and gain the nutrients necessary to attain the same fledging weights as early hatchers. This may be compounded by increased predation of chicks and kleptoparasitism of provisioning adults by great black-backed gulls and herring gulls ( Larus argentatus ) during the late breeding season, when both adult and juvenile gulls are foraging at the colonies. Consistent with this, Nettleship (1972) found that late hatching chicks fledge at a younger age on more level habitat, where chicks are more exposed to gull predation and adults are more vulnerable to kleptoparasitism. Research on this topic has repeatedly found reduced rates of predation and kleptoparasitism in gull-reduced or gull-free areas, but no resulting impact on offspring survival (Rice 1985, Finney et al. 2001, Finney 2002). Alternative growth strategies between early and late hatching chicks, where late hatchers develop quickly and fledge at younger ages but still maintain high survival, may partially explain this phenomenon. A follow-up to this work could investigate whether early and late hatchers truly experience differential predation/kleptoparasitism pressure, and whether these alternative strategies have long-term impacts on adult survival and/or reproductive success. This study provides new insights into the factors shaping and signaling offspring growth in a seabird with obligate biparental care. Several aspects of maternal colouration predicted puffling growth, providing support for female bill colour as a potential indicator of parental quality in this species. This may be a first glimpse at a potential explanation for the adaptive significance of the Atlantic puffin’s colourful bill and supports the notion that female ornamentation is not merely a byproduct of selection on males, but instead may play an important signaling role in its own right. However, the hatch timing of offspring was more consistently important in shaping puffling growth. The advantages of early vs. late breeding demonstrate a clear trade-off between weight gain and structural development, which may result from differential predation and kleptoparasitism across the breeding season. Whether the relationship between hatch timing and offspring growth holds in the face of climate change remains unknown but would provide a fruitful area of future research.

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Authors Metrics & Citations Metrics Article Usage 250views 156downloads Citations Download citation Katja Kochvar, Pierre-Paul Bitton. Hatch timing and maternal bill colouration are associated with chick growth in a mutually ornamented seabird. Authorea. 17 January 2025. DOI: https://doi.org/10.22541/au.173707203.31611712/v1 DOI: https://doi.org/10.22541/au.173707203.31611712/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu. Cited by - No evidence for assortative mating in the Atlantic Puffin, Journal of Ornithology, 167, 2, (387-397), (2025).https://doi.org/10.1007/s10336-025-02332-x Loading...