MULTIPLE SPATIAL AND CLIMATIC CONDITIONS AFFECT KINGBIRD FLYCATCHERS CLUTCH AND EGG SIZES

MULTIPLE SPATIAL AND CLIMATIC CONDITIONS AFFECT KINGBIRD FLYCATCHERS CLUTCH AND EGG SIZES | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 29 January 2025 V1 Latest version Share on MULTIPLE SPATIAL AND CLIMATIC CONDITIONS AFFECT KINGBIRD FLYCATCHERS CLUTCH AND EGG SIZES Authors : Marcelo Assis 0000-0002-9652-3628 [email protected] , Neander Heming 0000-0003-2461-5045 , and Miguel Marini Authors Info & Affiliations https://doi.org/10.22541/au.173814485.58742594/v1 Published Ecology and Evolution Version of record Peer review timeline 260 views 163 downloads Contents Abstract ABSTRACT KEYWORDS: INTRODUCTION METHODS RESULTS DISCUSSION DATA ACCESSIBILITY STATEMENT CONFLICT OF INTEREST STATEMENT TABLES FIGURES References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Variation in bird clutch and egg sizes across geographical gradients are issues often debated among ecologists, where latitudinal cline is a central question in several discussions. It is understood that these patterns are primarily driven by climatic conditions, where latitude acts as a proxy. Here, we achieve a robust dataset that covers a large territorial extent to test the hypothesis clutch and egg size will show measurable variation based on environmental gradients. We predict that these traits will: a) increase with increasing latitude, b) be larger in more seasonal climates (Köppen-Geiger), c) increase in sites experiencing cooler winters, and d) increase in sites with warmer and wetter long-term climatic conditions. We considered the geographically diverse Tyrannus genus (kingbirds) and collected breeding data from 35 scientific egg collections. After several data control processes, including spatial, temporal and taxonomic checking, we analysed the relationship between kingbird’s clutch and egg sizes with different climatic conditions. The analyses of 1358 clutches and 4750 eggs collected during 158 years (1858-2016) confirmed that Kingbirds’ clutch and egg sizes increase towards the poles. Both breeding traits varied according to main climates, regional sub-climates, and local temperature and precipitation conditions. More seasonal regions had the largest clutches, but sites with colder winters did not have the largest clutches. Tyrannus egg size increased in environments with less extreme dry periods. The significant relationship between larger eggs with sites with lower temperatures provides insights about the increase of egg size with latitude. Our findings suggest a robust correlation of residual variation in breeding traits with climatic conditions at both regional and local levels. Highly locally adapted species using climatic conditions as cues should also respond to interannual weather variations. The insights provided in this work can assist in understanding how species will cope with future climate scenarios. TITLE: MULTIPLE SPATIAL AND CLIMATIC CONDITIONS AFFECT KINGBIRD FLYCATCHERS CLUTCH AND EGG SIZES ABSTRACT Variation in bird clutch and egg sizes across geographical gradients are issues often debated among ecologists, where latitudinal cline is a central question in several discussions. It is understood that these patterns are primarily driven by climatic conditions, where latitude acts as a proxy. Here, we achieve a robust dataset that covers a large territorial extent to test the hypothesis clutch and egg size will show measurable variation based on environmental gradients. We predict that these traits will: a) increase with increasing latitude, b) be larger in more seasonal climates (Köppen-Geiger), c) increase in sites experiencing cooler winters, and d) increase in sites with warmer and wetter long-term climatic conditions. We considered the geographically diverse Tyrannus genus (kingbirds) and collected breeding data from 35 scientific egg collections. After several data control processes, including spatial, temporal and taxonomic checking, we analysed the relationship between kingbird’s clutch and egg sizes with different climatic conditions. The analyses of 1358 clutches and 4750 eggs collected during 158 years (1858-2016) confirmed that Kingbirds’ clutch and egg sizes increase towards the poles. Both breeding traits varied according to main climates, regional subclimates, and local temperature and precipitation conditions. More seasonal regions had the largest clutches, but sites with colder winters did not have the largest clutches. Tyrannus egg size increased in environments with less extreme dry periods. The significant relationship between larger eggs with sites with lower temperatures provides insights about the increase of egg size with latitude. Our findings suggest a robust correlation of residual variation in breeding traits with climatic conditions at both regional and local levels. Highly locally adapted species using climatic conditions as cues should also respond to interannual weather variations. The insights provided in this work can assist in understanding how species will cope with future climate scenarios. KEYWORDS: Biogeography, breeding traits, climate, clutch size, egg size, latitudinal effect, life-history. INTRODUCTION Like other seasonally adapted organisms, birds respond directly to environmental conditions through their life history parameters (Forchhammer et al., 1998; Kuleska, 1990). The condition of the environment acts as a driver of organisms’ physiology, and it can help explain variations in breeding traits (Bêty et al., 2003; Christians, 2002; Liu et al., 2018; Price and Liou, 1989). Climate, and the inherent climatic seasonality (White and Hastings, 2020), are key to bird phenology (Newton, 1998). Few bird groups reproduce year-round, even in tropical regions, where seasonality does not reach extreme conditions (Hau, 2001) . Migratory or not, birds must respond to climatic conditions at the breeding site (Murphy et al., 2022; Ockendon et al., 2013). Among the most evident clinal variations in ecology is the change in biological traits as a function of latitude (Hut et al., 2013). The relationship between clutch size and latitude is a strong pattern in reproductive investment where larger clutches are found at higher latitudes. This latitudinal pattern is also evident in animal body size (Stillwell, 2010), which itself is the main variation factor for birds’ clutch size (Jetz et al., 2008). Several assumptions have been proposed to explain this phenomenon (Lack, 1947; Ricklefs, 2000). However, the most accepted hypothesis is that the increase in clutch size is related to the seasonality of resources (Ashmole, 1963; Griebeler et al., 2004; Lundblad and Conway, 2021; Ricklefs, 1980). However, latitude is a proxy for the environmental conditions that exert pressure on the phenological adaptation of species, such as seasonality (Lundblad and Conway, 2021) and day length (Hut et al., 2013). Since seasonality provides increased resources per capita during the breeding season, better-nourished parents can invest in larger clutches. Besides changes in clutch size, explanations of the increase in egg size across geographical gradients are still unresolved. Many ecologists argue that decreasing egg sizes could compensate for the increase in clutch size (Blackburn, 1991), in which egg size would tend to decrease with increasing latitude. However, the positive relationship between egg and clutch sizes may indicate an environment with sufficient resource abundance for both reproductive investments to increase (Hõrak et al., 1995). Also, the positive relationship of eggs’ size with latitude may also be explained by the thermal properties of larger eggs, which lose heat more slowly than smaller eggs in colder environments (Martin, 2008). However, there are studies showing increased egg size with latitude (Stępniewski et al., 2021) while others found no relationship (Guo and Lu, 2022). Nonetheless, the variation of this trait is correlated with female body size (Bennett and Owens, 2002; Martin et al., 2006; Hermman Rahn et al., 1975) and incubation time (Hermann Rahn and Ar, 1974). Since body size of organisms within similar groups tends to be larger at higher latitudes (Blackburn et al., 1999; Stillwell, 2010), the latitudinal increase in adult body size could also be reflected in egg size and clutch size due to allometric relationships. Allometry explains about 80% of egg size variation (Blackburn, 1991). This ratio, considered a phylogenetic component, tends to maintain at high taxonomic levels, such as Order (Birchard and Deeming, 2015). The resulting variation not explained by the phylogenetic component, the residual variation, tends to be influenced by ecological and environmental factors (Martin et al., 2006). Once the environment pressures a trait, it can respond independently of a trade-off (Blackburn, 1991). Resource richness and abundance may be the essential factors in fine-tuning the timing of reproduction when females set the laying and hatching season so that the time of greatest energetic demand by the offspring coincides with the expected peak of resource availability (Dunn and Winkler, 2008; Welty, 1982). Although studies have already diagnosed some of the residual variations in egg size – clutch size (Blackburn, 1991; Martin, 2008; Martin et al., 2006; Smith and Fretwell, 1974), environmental conditions (Heming and Marini, 2015; Martin, 2008; Martin et al., 2006), migratory behaviour (Heming and Marini, 2015; Sousa et al., 2024) and geographical variation (Martin et al., 2006) – these variables still do not account for all the residual variation (Martin, 2008; Martin et al., 2006). Hence, there is still a need to know how breeding traits can vary within a group of birds over time and space (Heming and Marini, 2015; Murphy et al., 2020). We used a geographically widespread tyrant flycatcher genus and collected breeding data from scientific egg collections. This dataset covers their entire latitudinal range throughout almost all the American continents. This work aims to evaluate how residual variations in clutch and egg size of Kingbirds ( Tyrannus species), in addition to well-established ones such as allometry, are influenced by geo-climatic characteristics. We aim to answer how clutches and egg sizes vary according to 1) latitude; 2) main climate (Köppen-Geiger climate classification; ”KG”); 3) subclimates within main KG climate classification; and 4) local long-term averages, i.e., historical climatic conditions based on temperature and precipitation parameters. Accordingly, testing the hypothesis that clutch and egg size will show measurable variation based on environmental gradients, we predict that these traits will a) increase with increasing latitude, b) be larger in more seasonal climates (Köppen-Geiger), c) increase in sites experiencing cooler winters, and d) increase in sites with warmer and wetter long-term climatic conditions. METHODS Database In scientific egg collections, we collected reproductive data from Kingbird species ( Tyrannus genus). We choose the collections by researching and accessing the digitised and tabulated collections on their websites or scientific collection aggregators ( e.g ., GBIF, VertNet). With the knowledge of the contents of most part of these collections, we visited 35 museums in South America, the USA, and Europe. We took a careful visual approach and inspected the eggs to confirm the main characteristics of the Tyrannidae family. We excluded egg sets with eggs of abnormal colour, mark patterns, or size for a given species. Then we checked the data shown on the respective labels/cards. With prior knowledge of the breeding biology of each species, we excluded clutches that did not accurately reflect the final clutch size, due to predation, parasitism, or collecting bias. We only proceeded with those egg sets whose clutch size was reliable, even for the egg size analyses. In addition to clutch size, the records provided laying location and date, which was crucial to verify the data’s integrity with information on each species natural history patterns, matching the location with species distributions (BirdLife International, 2020) and the laying date with their recorded phenology. The first taxonomic classification, whenever available, was essential to review taxonomic resolution, through synonymy, for the outdated taxonomic classifications of some collections. The taxonomic resolution for Tyrannus species followed the order: Sclater (1888), then Cory and Hellmayr (1927), Amadon et al. (1979), Phillips 1994), and finally, we updated the species names (and subspecies when possible) following eBird/Clements check-list (Clements et al., 2021). We took pictures of egg clutches arranged on a black base with a ruler scale and their respective collection labels/cards (Supporting information Figure S1). We extracted egg dimensions from each picture using ImageJ software (Schneider et al., 2012) associated with the EggTools add-on (Troscianko, 2014). Besides the main dimensions (length, width, perimeter, and area), the process allowed us to extract eggs volume (mm³), which we treat here as ”egg size” (Supporting information Figure S2). Establishing a maximum error of ±25km, we used the clutch collection site present in the cards to obtain the geocode (decimal latitude and longitude) through the ggmap package, established through the centroid of the locality, defined by Google Maps service (Kahle and Wickham, 2013). Every locality that did not contain a geocode but provided localities with errors within a ±25km limit was integrated into our database. This error was defined to be compatible with the respective climate information. With this level of coordinates resolution, we obtained climate conditions at the locality of egg-laying. To classify the climate where clutches were collected, we used the geo-climatic raster model of Köppen-Geiger (Kottek et al., 2006), available in shapefiles, according to its geographic position from each clutch geocode. The combination of the five main climates (A – Equatorial climates, B – Arid climates, C – Warm temperate climates, D – Snow climates, and E - Polar climates) and temperature and precipitation parameters resulted in 31 possible climates ( e.g. , Af – Equatorial rainforest, fully humid; Csa - Warm temperate climate with dry and hot summer and summer; Table 1, Fig. 1). Equatorial climates (A) are characterised by high temperatures throughout the year in comparison with other climates, with average annual minimum temperatures ranging around 18 °C. Arid climates (B) are characterised by droughts with threshold precipitation below 10 mm. Warm temperate climates (C) are characterised by average annual minimum temperatures ranging from -3 to 18 °C. Snow climates (D) are classified by their average minimum temperature of around -3°C. Finally, Polar climates (E) are characterised by average maximum temperatures of less than 10 °C. The data is made available digitally on the website http://koeppen-geiger.vu-wien.ac.at (Rubel et al., 2017). Its climate models include historical temperature and precipitation indices with a monthly resolution of the entire land area of the planet, with a spatial resolution of 0.5 degrees latitude/longitude (Kottek et al., 2006). To analyse effects of local temperature and precipitation conditions we used breeding records from 1901-2016, and the local historical temperature and precipitation variables from WorldClim Bioclimatic Database (Fick and Hijmans, 2017) at a 10-minute resolution to identify the local climate characteristics. We extracted the values of the following nine bioclimate variables: isothermality (T iso ), temperature seasonality (T sea ), maximum temperature of warmest month (T max ), minimum temperature of coldest month (T min ), annual mean temperature (T myr ), annual precipitation (P ryr ), precipitation of wettest month (P max ), precipitation of driest month (P min ), and precipitation seasonality (P sea ), also using the geocode of the clutch collection site. Statistics We analysed the data using Phylogenetic Generalized Linear Mixed Models (PGLMMs). Since we aimed to observe geo-climatic variation across the entire Tyrannus genus, we set the species taxon as a random variable for clutch and egg size analyses. To control for the phylogenetic effect among related species, we gathered phylogenetic trees of Tyrannus species as a subset of the Global Phylogeny of Birds (Jetz et al., 2012). We downloaded 1,000 possible phylogenetic trees (Ericson stage 2 backbone) and calculated a single consensus tree summarizing the most common using “ape” (Paradis and Schliep, 2019) and “phangorn” (Schliep, 2011) packages. Exclusively for egg size, and recognising intraclutch dependence (Christians, 2002), we used clutch identity to analyse climate effects on kingbirds’ egg size (random slope). The analyses were divided into four geo-climatic scales: from largest to smallest – latitude, main climate, subclimate, and local historical index of temperature and precipitation. For each geo-climatic scale, a pair of analyses were performed for clutch and egg size as dependent variables. The latitudinal effect on clutch and egg size was evaluated using the absolute value of latitude, from the clutch collection site, as a fixed variable. Then, we assessed the effect of climate conditions on the reproductive investment of Kingbirds in two other steps. The effect of climate was analysed using the main climates as a fixed variable, and then, within each main climate, we analysed the effect the subclimate had on the breeding traits, considering its characteristics among precipitation and temperature conditions. We evaluated the smaller-scale climate effect by combining the local bioclimatic indices as fixed variables. To avoid multicollinearity among climate variables, we first estimated the pairwise Pearson’s correlation coefficients and judged the high correlations (|r| > 0.7). Once we detected a correlation between a pair of variables, we excluded one of those, keeping the variable we considered as most important for the model. An initial model was established with all fixed variables. Then, assessing the weight and importance of each variable for the model using Deviance Information Criterion (DIC), we fitted the models by backward deletion, excluding less weighed variables to build the best-fixed structure. All climatic continuous variables adopted were standardised with a mean of zero and a standard deviation of one. Finally, we checked for overdispersion for each model, where no overdispersion was detected. All these steps, from the construction of the dataset to the analyses, were performed using the software R (R Core Team, 2024) where we used the ”MCMCglmm” package (Hadfield, 2010) to analyse the PGLMM. RESULTS Database We assembled breeding records of 2931 clutches and 9529 eggs for all 13 Kingbird species. We were able to take pictures from 1657 clutches in the egg collections. After checking the taxonomy, consistent species-clutch sizes, and geographic distribution, we trimmed the dataset to 1358 clutches and 4750 eggs (Supporting information Image S3). Then, after observing the discrepancy in the number of clutches of each species, we chose to proceed with the analyses with only eight species, removing the clutches of: Tyrannus niveigularis, T. crassirostris, T. cubensis, T. albogularis, and T. caudifasciatus . Together, these five species totalled only 36 clutches and 70 eggs. Therefore, we analysed 1332 clutches and 4680 measured eggs from Tyrannus melancholicus , T. savana , T. dominicensis , T. tyrannus , T. verticalis , T. couchii , T. vociferans, and T. forficatus (Supporting information Table S1; Supporting information Image S4), which covered the entire latitudinal extent of the genus’ distribution in the American continents as well as presence in four main climates and 17 subclimates (Supporting information Table S2). Statistics The PGLMM revealed that clutch size had a significant and positive variation with absolute latitude (β = 0.02 ± 0.003, Table 1, Fig. 2A). The clutch size of kingbirds also varied among the main climates (Table 2, Fig. 3A). Taking the Equatorial climates as the intercept, where the clutches were smaller, clutch sizes were larger in all other climates. Among all the main climates, the Snow climate had the largest clutches, followed by Warm Temperate and Arid climates. Kingbirds’ egg size also increased with latitude (β = 0.01 ± 0.002, Table 1, FIG. 2B). Like clutch size, egg size was smaller in Equatorial climates and larger in Snow climates, followed by Warm Temperate climates (Table 2, Fig. 3B). Although, there was no significant difference between egg size among climates in Equatorial and Arid climates. In the climates, there was a significant variation in clutch size among Equatorial subclimates (Table 3). The clutches were larger (Figure 4A), and the eggs were smaller (Figure 5A) in the As subclimate, that is characterised by dry summers with minimum precipitation of less than 60 mm. Eggs were also slight smaller in the Aw subclimate than the Af subclimate (Figure 5A), where the major characteristics are dry winters, with average minimum precipitation of less than 60 mm. Only egg size had significant variation among arid subclimates (Table 3). Kingbird eggs were larger in the BSk and even larger in the BWk sub-climate (Figure 5B). These two Arid climates have in common the recording of average annual temperatures below 18°C and differ in precipitation rates. While BSk has a precipitation threshold between 5 and 10 mm, in the BWk climate, species experience a precipitation threshold of less than 5 mm, characterising the region as desert. In Warm temperate subclimates, clutch size was significantly smaller in Cfb and Csb climates (Table 3, Figure 4C), which are characterised by dry summers, with higher precipitation rates in winter and at least four months with average temperatures between 10 and 22 °C. Among these sub-climates, egg sizes were larger in the Cfb but smaller in the Cwb , characterized by dry winters and warm summers, followed by Csa subclimate, characterized by dry and hot summers (Table 3, Figure 5C). In Snow climates, climate Dfb showed the largest clutches (Table 3, Figure 4D), and climates Dfb and Dfc had eggs significantly larger (Figure 5D). In common, these climates have high humidity and high precipitation and differ in summer temperatures. While Dfb has a warm summer, Dfc has a cool summer. The PGLMM best-adjusted for clutch size was composed of three climatic variables (TABLE 4), including isothermality (β = -0.40 ± 0.04; FIGURE 6A), seasonality of precipitation (β = 0.17 ± 0.02; FIGURE 6B), and minimum temperature of the coldest month (β = 0.11 ± 0.04; FIGURE 6C). In this model, all variables had significant coefficients. This model indicated that kingbirds’ clutch size is larger in locations with less stable annual temperatures, higher seasonal precipitation, and the coldest month of the year with higher minimum temperatures. For egg size, the best-adjusted PGLMM had three variables (TABLE 4), including precipitation of the driest month (β = 0.08 ± 0.01; FIGURE 7A), maximum temperature of the warmest month (β = -0.08 ± 0.01; FIGURE 7B), and minimum temperature of the coldest month (β = -0.11 ± 0.01; FIGURE 7C). All three variables that compound the model were significant. These model coefficients show that kingbirds’ egg sizes are larger where the year’s driest month has more rainfall and tends to be smaller where the warmest month has higher maximum temperatures and the coldest month has higher minimum temperatures. DD MMMM YYYY \acceptedDD MMMM YYYY DISCUSSION Kingbirds clutches were larger at sites with higher rainfall seasonality (P sea ) and thermal variation (T iso ). It is already agreed that species inhabiting more seasonal environments tend to have larger clutches, especially related to temperature variation (Jetz et al., 2008; Stevens, 1989). The increase in seasonality with latitude is one of the most accepted explanations for bird clutch size (Ashmole, 1963; Lundblad and Conway, 2021). Ashmole’s hypothesis considers that harsher winter conditions tend to leave individuals more vulnerable and susceptible to starvation, but also to hypothermia, decreasing population density at the breeding season. However, the temperature variability parameter relevant for clutch size was not temperature seasonality (T sea ) but isothermality (T iso ). While temperature seasonality is based on average temperatures, isothermality uses monthly and annual ranges, with maximum and minimum temperatures (O’Donnell and Ignizio 2012). Extreme temperatures have been a better predictor than average temperatures (Schaper et al., 2012) because temperature parameters have a higher potential to cause physiological stress. Our results also show that clutches tend to be larger in environments with higher precipitation seasonality, corroborating Ashmole’s hypothesis. As for rainfall, our data contradicted the prediction. Precipitation is an important primary productivity factor and crucial for increasing invertebrate populations (Pinheiro et al., 2002). However, it important to stress two aspects: (1) the reproductive events of birds are highly dependent on invertebrates as a protein source for egg development (McWilliams et al., 2016); and (2) Tyrannus is a tyrant flycatcher genus of primarily insectivorous flycatchers (Fitzpatrick 1980, Murphy 1983b), which tends to correlate its reproductive output with insect abundance (Blancher and Robertson, 1987; Murphy et al., 2022). Low seasonality of precipitation keeps insect populations at equilibrium throughout the year (Wolda, 1988). However, kingbirds had larger egg sizes at sites with higher precipitation in the drier months of the year (P min ). This means that more pronounced droughts can affect egg size for some species, even if not occurring in the reproductive season. Given that larger eggs produce larger chicks, one would expect that more benign breeding seasons would allow the production of higher-fitness offspring (Krist, 2011). Additionally, in Equatorial climate, kingbirds tend to have larger clutches in subclimates with dry summers (As). However, it is necessary to understand how egg composition, in addition to their size, can also ensure chick quality (Birchard and Deeming, 2015). This result suggests a more direct relationship between precipitation and egg size. Furthermore, a time lag effect of the weather on breeding traits seems important for some species and deserves further consideration. Sites with warmer winters, with higher minimum temperatures (T min >), had larger clutches and smaller eggs. As well as for local parameters, subclimates with colder winter temperatures also presented smaller clutches (Csb). Breeding sites with higher maximum temperatures in the warmest month (T max ) also had smaller eggs. Combined with this result, the arid subclimates where kingbirds’ eggs were largest were those with the lowest annual temperatures (BSk, BWk). The high energy expenditure of laying birds may increase under both extreme cold and extreme hot conditions. In warmer environments, the thermal potential can lead to bird dehydration, mainly in water-limited environments, such as arid regions (Sauve et al. 2021, Schifferli et al. 2014, Whitfield et al. 2015). However, in warm-humid environments, productivity tends to lead to higher insect availability (Grüebler et al., 2008), the main food resource of kingbirds. Associating these lines of thought helps us understand that there is a complex balance between resource supply and thermal potential, which is crucial for future ecological inquiries. In Snow climates, Tyrannus only breeds in wet subclimates (Df*), but larger clutches occur only in subclimates that have warm summers (Dfb), while larger eggs are found in subclimates that have cool summers (Dfc). The high precipitation in this last subclimate, combined with the increased egg size in environments with more rain in the driest month of the year (Pmin>), also shows the importance of rainfall for kingbird’s egg size, even though other studies have found no significant effect of precipitation for one kingbird species (Murphy, 1983b). Even if precipitation occurs during winter, climatic events tend to influence individuals and populations in subsequent seasons, supporting the importance of the time lag effect (Marra et al., 2015). Kingbirds’ clutch sizes tended to be larger in Snow climates, followed by Warm temperate and Arid climates, and were smaller in Equatorial climates, though with confidence interval overlap and lack of difference among some main climates. This progression does not coincide with summer temperatures or precipitation but coincides with a decrease in winter temperatures. Higher latitude species tend to cover a higher latitudinal range and be better tolerant to temperature variation (Stevens, 1989). In addition, local and regional sites with lower minimum winter temperatures tend to have smaller clutches. The progression of egg sizes followed the trend observed for clutch size, and Snow climates had the largest eggs. Particularly for egg size, Equatorial and Arid climates did not differ. Our findings so far indicate a combination of local factors that underlie the geographic variation in kingbird breeding traits. The thermal potential of colder environments subjects female kingbirds, as a unique parental incubator species (Blancher and Robertson, 1985; Murphy, 1996), to greater difficulties in maintaining optimum incubation temperature (Gillette et al., 2021), generating greater energy expenditure. Thus, the energy expenditure for heating and re-heating eggs in cold environments appears to be even higher than in warm environments. The premise that the low availability of resources would lead to a decrease in the physiological conditions of the females, who would have lower body mass and consequently lay smaller eggs (Järvinen and Ylimaunu, 1986), can be contrasted with the temperature indices. Larger chicks from larger eggs may show greater tolerance and resistance to heat loss in colder environments (Krist, 2011). Larger eggs, which ensure greater offspring survival, when associated with an after-hatching attendance increase, must compensate for the lower resource conditions in cooler than in warmer environments (Sabine Gebhardt-Henrich and Richner, 1998). Furthermore, larger eggs have better heat conservation in cooler climates, allowing females more time outside the nest foraging (Gillette et al., 2021). Since kingbirds eggs are larger in response to colder conditions, we support the idea that this balance could guarantee better survival chances for the females, with an advantageous impact on their survival soon. The latitudinal variation in kingbird’s clutch size corroborates again the pattern observed since Moreau (1944), increasing significantly toward the poles (Cody, 1966). Environments with more seasonal climates and temperature variations have larger clutches (Griebeler et al., 2004), and climate seasonality was a common factor among the climates and climatic conditions that allowed kingbirds to increase their clutches at higher latitudes. However, harsher winter conditions do not promote larger clutches. Thus, even if winters are harsh and decrease population density (Lv et al., 2023), and increase per capita resource availability during the breeding season, the quality of resources in these localities does not provide the energy demands to increase clutch size (Ockendon et al., 2013). Egg size followed the same trend as clutch size and increased with latitude. As an important factor for primary productivity, regions with milder winters can reflect a reproductive season with greater resource abundance, but also high competitiveness (Newton, 1998) . The progression of egg size with latitude appears to be negatively related to temperatures, as observed for other vertebrate groups (Feiner et al., 2016; Sheader, 1996). Higher latitudes share the climatic characteristics that enable kingbirds to lay larger eggs. As the explanation of life history patterns is based on complex interactions of traits (Bennett and Owens, 2002) and trade-offs based on allocation of resources (Stearns, 1992), the investment in clutch size can be correlated with adjustment in egg size (Roff and Fairbairn, 2007; Stearns, 1992), which depends on the short-term strategy females will adopt when meeting the environmental conditions in the reproduction cycle (Aranzamendi et al., 2019) . Tyrannus , as a genus, lacks a trade-off pattern between clutch and egg sizes, varying among species in positive, negative, and no apparent trade-off. A trade-off in these traits tends to be a species-specific characteristic of Tyrannus and does not appear to be a general rule since most species already assessed do not show a correlation between clutch and egg size (Christians, 2002; Sakai, 2021). In addition to the direct parental effect on clutch and egg size, much of the variation of an offspring trait is due to environmental quality. Tyrannus tyrannus , for example, changes its reproductive performance interannually as a function of environmental quality (Blancher and Robertson, 1985). Since species adapt their reproductive traits to a given climate, interannual variation in climatic conditions tends to pressure the plasticity of their traits (Visser, 2008) . Kingbirds have breeding characteristics that are dependent on climatic conditions. Members of this genus have high fidelity to the breeding site (Blancher and Robertson, 1985; Murphy, 1996) and select habitats based on climatic parameters, as is the case for T. savana and T. tyrannus , where temperature is a cue for breeding sites (MacPherson et al., 2018) and T. savana uses precipitation to select wintering sites (Jahn et al., 2013). In addition, heritability reinforces the relationship between species and local climate across generations (Christians, 2002). It is essential, therefore, to know how climates define species’ reproductive traits and how they respond to variations in climatic conditions. Climate unpredictability is one of the most significant factors experienced by species through global climate change (Hansen et al. 2012), with negative effects on their fitness (McNamara et al., 2011). Kingbirds and other species are more responsive to extreme than average temperatures (Schaper et al., 2012). Global climate change will most impact extreme climate parameters (Marcelino et al., 2020). Heat waves, droughts, and excessive rainfall are consequences of climate change and can affect birds depending on which stage of reproduction they experience these conditions (Cady et al., 2019; Sauve et al., 2021). Projections of future climate change scenarios show that birds are more vulnerable to future thermal stresses, even more than mammals (Riddell et al., 2021). In conclusion, our findings show that kingbirds clutch and egg sizes vary according to regional and local climate, and that these correlations shed light on the latitudinal cline of reproductive investment. Still, it is not possible to assert whether the variation in investment in reproduction is a life history strategy or a physiological response. Therefore, more research relating climatic conditions to life history traits is indispensable. Importantly, there is a recognition that species from different biogeographic regions may have shared or divergent responses to climatic conditions and that responses of one species restricted to the Nearctic region are not necessarily similar to those by a Neotropical species. Furthermore, long-term and longitudinal data is essential to understand the effects of global climate change on species and how their plasticity will buffer future climatic conditions. DATA ACCESSIBILITY STATEMENT Data available from the Dryad digital repository: https://doi.org/10.5061/dryad.qz612jmmd For peer review: https://datadryad.org/stash/share/0EPYaXcFHvUfyYRGaVUsirIWXWtVHdEZy4BIf8dqjWU CONFLICT OF INTEREST STATEMENT Conflict of Interest statement - ’Not applicable.’ Ethics approval statement - ’Not applicable.’ Patient consent statement - ’Not applicable.’ Permission to reproduce material from other sources - ’Not applicable.’ Clinical trial registration - ’Not applicable.’ TABLES Table 1 - A summary of the Phylogenetic Generalized Linear Mixed Models investigating latitudinal variation in clutch and egg sizes of kingbirds. Egg size was standardised with a mean of zero and a standard deviation of one. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Predictors Estimates SE CI Estimates SE CI Intercept 2.99 ** 0.54 -1.92 – 4.11 -0.61 1.01 -2.61 – 1.51 Absolute latitude 0.02 *** 0.003 0.01 – 0.03 0.01 *** 0.002 0.00 – 0.01 * p<0.05 ** p<0.01 *** p<0.001 Table 2 - A summary of the Phylogenetic Generalized Linear Mixed Models investigating variation in clutch and egg sizes of kingbirds as a function of the main Koppën-Geiger main climates. Egg size was standardised with a mean of zero and a standard deviation of one. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Clutch size Egg size Predictors Estimates SE CI Estimates SE CI [A] Equatorial climates (Intercept) 3.56 *** 0.49 2.52 – 4.48 -0.30 0.97 -2.27 – 1.68 [B] Arid climates 0.26 *** 0.08 0.11 – 0.42 0.06 0.03 -0.02 – 0.14 [C] Warm Temperate climates 0.35 *** 0.06 0.23 – 0.47 0.17 *** 0.03 0.10 – 0.23 [D] Snow climates 0.41 ** 0.11 0.18 – 0.63 0.33 *** 0.05 0.23 – 0.44 * p<0.05 ** p<0.01 *** p<0.001 Table 3 - A summary of the Phylogenetic Generalized Linear Mixed Models investigating variation in clutch and egg sizes of kingbirds as a function of the Koppën-Geiger subclimates. Egg size was standardised with a mean of zero and a standard deviation of one. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Predictors Estimates SE CI Estimates SE CI Equatorial climates Af (Intercept) 3.50 ** 1.04 1.64 – 5.73 -0.06 1.46 -2.58 – 2.69 Am 0.14 0.16 -0.18 – 0.45 -0.04 0.09 -0.23 – 0.15 As 1.27 *** 0.31 0.67 – 1.87 -0.66 *** 0.15 -0.97 – -0.36 Aw 0.14 0.14 -0.13 – 0.43 -0.32 *** 0.08 -0.50 – -0.15 Arid Climates BSh (Intercept) 3.80 *** 0.50 2.87 – 4.82 -0.39 1.03 -2.18 – 1.69 BSk 0.02 0.12 -0.22 – 0.26 0.21 *** 0.04 0.13 – 0.30 BWh 0.28 0.25 -0.21 – 0.80 -0.12 0.09 -0.29 – 0.06 BWk -0.35 0.29 -0.91 – 0.23 0.55 *** 0.10 0.34 – 0.78 Warm Temperate climates Cfa (Intercept) 3.99 *** 0.45 3.06 – 5.00 -0.14 1.19 -2.48 – 2.00 Cfb -0.45 * 0.22 -0.90 – -0.01 0.31 ** 0.11 0.10 – 0.53 Csa -0.31 0.16 -0.63 – 0.01 -0.19 ** 0.07 -0.34 – -0.04 Csb -0.35 * 0.16 -0.67 – -0.03 0.12 0.07 -0.03 – 0.26 Cwa -0.03 0.14 -0.29 – 0.26 0.07 0.07 -0.07 – 0.22 Cwb 0.15 0.39 -0.56 – 0.93 -0.64 *** 0.20 -1.04 – -0.25 Snow Climates Dfa (Intercept) 3.48 * 1.23 2.82 – 4.33 -0.15 2.10 -1.93 – 1.22 Dfb 0.52 * 0.20 0.12 – 0.91 0.25 ** 0.08 0.08 – 0.42 Dfc 0.36 0.35 -0.29 – 1.10 0.64 *** 0.16 0.32 – 0.96 * p<0.05 ** p<0.01 *** p<0.001 Table 4 - Phylogenetic generalized linear mixed models, adjusted by the backward selection, for clutch and egg sizes and long-term mean parameters of precipitation and temperature. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Continuous variables were standardised with a mean of zero and a standard deviation of one. \received DD MMMM YYYY \acceptedDD MMMM YYYY Predictors Estimates SE CI Estimates SE CI Intercept 3.79 *** 0.38 3.10 – 4.61 -0.19 0.96 -2.13 – 1.58 Isot -0.40 *** 0.04 -0.49 – -0.32 - - - Pmin - - - 0.08 *** 0.01 0.05 – 0.10 Psea 0.17 *** 0.02 0.11 – 0.22 - - - Tmax - - - -0.08 *** 0.01 -0.10 – -0.06 Tmin 0.11 * 0.04 0.03 – 0.19 -0.11 *** 0.01 -0.13 – -0.07 * p<0.05 ** p<0.01 *** p<0.001 FIGURES Figure 1 - Map of the 31 Köppen-Geiger subclimates of the Americas divided into five main climates (A – Equatorial climates, B – Arid climates, C – Warm temperate climates, D – Snow climates, and E – Polar climates) generated from observed temperature and precipitation data from 25 years (1986-2010). Source: Adapted from http://koeppen-geiger.vu-wien.ac.at Figure 2 – Relationships between clutch size (A) and egg size (B) of kingbirds and absolute latitude by Phylogenetic Generalized Linear Mixed Models (PGLMM). Figure 3 - Variation in the clutch (A) and egg sizes (B) of kingbird species in Equatorial (A), Arid (B), Warm temperate (C), and Snow (D) climates classified from (Kottek et al., 2006). Egg size variation was standardised with a mean of zero and a standard deviation of one. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Figure 4 - Variation in the clutch size of kingbird species in subclimates of Equatorial (A), Arid (B), Warm temperate (C), and Snow (D) climates, classified by (Kottek et al., 2006). Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Figure 5 - Variation in the egg size of kingbird species in subclimates of Equatorial (A), Arid (B), Warm temperate (C), and Snow (D) climates, classified by (Kottek et al., 2006). Egg size variation was standardised with a mean of zero and a standard deviation of one. Significant effects are classified by p-value (* p<0.05; ** p<0.01; *** p<0.001). Figure 6 - Relationships between clutch size of kingbirds and climatic parameters by Phylogenetic Generalized Linear Mixed Models (PGLMM). The backward selection resulted in the model with the variables: (A) isothermality (T iso ), (B) precipitation seasonality (P sea ), and (C) minimum temperature of the coldest month (T min ). Figure 7 - Relationships between egg size of kingbirds and climatic parameters by Phylogenetic Generalized Linear Mixed Models (PGLMM). 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Vol. 19 , (157), 1–18. https://doi.org/10.1146/annurev.ecolsys.19.1.1 Crossref Google Scholar Information & Authors Information Version history V1 Version 1 29 January 2025 Peer review timeline Published Ecology and Evolution Version of Record 19 Apr 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords biogeography climate clutch size egg size latitudinal effect life-history Authors Affiliations Marcelo Assis 0000-0002-9652-3628 [email protected] Universidade de Brasília View all articles by this author Neander Heming 0000-0003-2461-5045 Universidade Estadual de Santa Cruz View all articles by this author Miguel Marini Universidade de Brasilia View all articles by this author Metrics & Citations Metrics Article Usage 260 views 163 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Marcelo Assis, Neander Heming, Miguel Marini. MULTIPLE SPATIAL AND CLIMATIC CONDITIONS AFFECT KINGBIRD FLYCATCHERS CLUTCH AND EGG SIZES. 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