Investigating diurnal effects and joint nest defense behavior in Herring Gulls

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Shah , Maddie E. Ellms , Frances Dygean , Scarlett Gunasekera , Emily Hart , Reka E. Ivanyi , Morgan Lane , David Liu , DeAnna Pitcher , Abigail Saucier , Kathleen F. Schroeder , Joseph Triquet , Kit M. Williams , View ORCID Profile Kristen M. Covino doi: https://doi.org/10.1101/2025.06.13.659635 Shailee S. Shah 1 Cornell Lab of Ornithology , Ithaca, NY, USA 2 Ecology and Evolutionary Biology Department, Cornell University , Ithaca, NY, USA 9 Shoals Marine Laboratory, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Shailee S. Shah For correspondence: sss253{at}cornell.edu Maddie E. Ellms 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Frances Dygean 4 Biology Department, Loyola Marymount University , Los Angeles, CA, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Scarlett Gunasekera 5 College of Arts and Sciences, Cornell University , Ithaca, NY, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Emily Hart 5 College of Arts and Sciences, Cornell University , Ithaca, NY, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Reka E. Ivanyi 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Morgan Lane 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site David Liu 6 College of Agriculture and Life Sciences, Cornell University , Ithaca, NY, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site DeAnna Pitcher 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Abigail Saucier 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Kathleen F. Schroeder 7 Biology Department, Environmental Studies Program, Middlebury College , Middlebury VT, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Joseph Triquet 3 College of Life Sciences and Agriculture, University of New Hampshire , Durham, NH, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Kit M. Williams 8 Southern Maine Community College , South Portland, ME, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site Kristen M. Covino 4 Biology Department, Loyola Marymount University , Los Angeles, CA, USA 9 Shoals Marine Laboratory, University of New Hampshire , Durham, NH, USA 10 Gulls of Appledore Research Group , Appledore Island, ME, USA Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Kristen M. Covino Abstract Full Text Info/History Metrics Data/Code Preview PDF Abstract Nest defense in birds is vital for the protection of their young, but can prove energetically costly. Birds often show plasticity in nest defense depending on factors such as threat level, mate presence, nest stage, and body condition. Such factors can vary over temporal extents ranging from a diel cycle to a breeding season or an individual’s lifetime. Understanding diurnal variation in nest defense intensity can be particularly useful when studying a breeding population to help investigators minimize nest disturbance, yet few studies have explored diurnal variation in nest defense intensity. Here we investigated how nest defense by Herring Gulls ( Larus smithsonianus ) varies with time of day. We simulated predatory threat at 26 Herring Gull nests during four different times of day: “Early Morning” (0530-0600), “Late Morning” (1000-1030), “Afternoon” (1400-1430), and “Evening” (1900-1930). Based on previous findings of diurnal activity patterns in a Herring Gull, we predicted that nest defense intensity—measured as aggressiveness of response and latency to calm—would be greater during the early morning and late evening than at other times. Contrary to our prediction, we found that the time of day did not affect nest defense intensity in Herring Gulls. However, independent of time of day, we found that when both mates were present at the nest, aggression scores were elevated. Our results suggest that joint nest defense in Herring Gulls permits greater aggressiveness towards predators, perhaps due to division of labor or lowered risk of complete nest failure if one parent is injured or killed. Further, our results indicate that researchers can minimize nest disturbance and accompanying stress by limiting research activities at Herring Gull nests when both parents are present. Introduction Nest defense, defined as “a behavior that decreases the probability that a predator will harm the contents of the nest (eggs or chicks) while simultaneously increasing the probability of injury or death to the parent” ( Montgomerie & Weatherhead, 1988 ), is a costly behavior which can vary in intensity depending on various factors that influence its relative costs and benefits ( Caro, 2005 ). Previous studies in a variety of avian species have shown that the intensity of nest defense varies based on internal factors such as parental sex ( Brunton, 1990 ; Galeotti et al., 2000 ; Kryštofková et al., 2011 ; Paredes et al., 2006 ; Redmond et al., 2009 ; Reyer et al., 1998 ; Rytkönen et al., 1993 ; Sproat & Ritchison, 1993 ; Tkaczyk et al., 2022 ; Włodarczyk & Minias, 2015 ) and experience ( Knight & Temple, 1986 ; Thornhill, 1989 ) as well as external factors such as offspring age and vulnerability ( M. Brown & Brown, 2004 ; Crisologo & Bonter, 2017 ; Pavel & Bureš, 2001 ; Redmond et al., 2009 ; Redondo & Carranza, 1989 ; Regelmann & Curio, 1983 ; Strnadová et al., 2018 ; Whittam & Leonard, 2000 ), presence and behavior of mate ( Chase, 1980 ; Crisologo & Bonter, 2017 ), and mating system ( Larsen, 1991 ). However, though birds often show distinct diurnal cycles in activity patterns ( Robbins, 1981 ), few studies have examined how nest defense varies with time of day ( Bukacińska & Bukaciński, 1994 ; Burger, 1981 ). Birds show distinct diurnal patterns in activities such as territorial singing and defense ( Bukacińska & Bukaciński, 1994 ; Burger, 1981 ; Galusha & Amlaner, 1978 ; Jahn et al., 2017 ; Moran et al., 2019 ; Searfoss et al., 2020 ; Staicer et al., 2019 ), foraging ( Capilla-Lasheras et al., 2024 ; Dismas et al., 2021 ; Kelly & Wood, 1996 ; Rastogi et al., 2006 ; Regular et al., 2010 ), movement and habitat use ( Galusha & Amlaner, 1978 ; Harmange et al., 2021 ; Klarevas-Irby & Farine, 2024 ), incubation ( Drent, 1970 ; Rastogi et al., 2006 ), mate guarding and copulation ( Burger, 1976 ; Galusha & Amlaner, 1978 ), and antipredator responses ( Ferguson et al., 2019 ; Metcalfe & Ure, 1997 ). These patterns are in response to diurnal patterns of factors such as energetic condition ( Ferguson et al., 2019 ; Metcalfe & Ure, 1997 ), circulating hormone levels ( Eikenaar et al., 2020 ; Ramenofsky et al., 1999 ; Schwabl et al., 2016 ), ambient temperature ( M. Brown & Brown, 2004 ), and activity of predator and prey species ( Drent, 1970 ; Galusha & Amlaner, 1978 ; Harmange et al., 2021 ; Regular et al., 2010 ; Ribic et al., 2021 ). Such factors may be particularly important in colonial nesting seabirds that often forage many miles away from their nest, experience diurnal variation in prey availability, and face nest predation pressure from other individuals in the colony ( Schreiber & Burger, 2002 ). For example, in a Black Skimmer ( Rynchops niger ) colony, intraspecific aggression levels showed a daily biphasic pattern, peaking in the early morning and late afternoon in conjunction with colony-level disturbances ( Burger, 1981 ). Similarly, in a Black-headed Gull ( Larus ridibundus ) colony, intraspecific aggression peaked in the late afternoon to coincide with increased colony activity as birds departed to and arrived from foraging sites ( Bukacińska & Bukaciński, 1994 ). Seabirds are also widely studied in their breeding colonies as indicator species for marine ecosystems ( Piatt et al., 2007 ). Understanding fine-scale variation in nest defense patterns is thus vital to minimize distress to breeding seabirds, and its potential downstream effects ( McNew et al., 2024 ; Riou et al., 2010 ; Siller Wilks et al., 2024 ), while conducting research involving handling eggs or chicks. However, to our knowledge, no study has examined variation in nest defense with time of day using an experimental approach. Here, we examined diurnal patterns in nest defense intensity in nesting Herring Gulls ( Larus smithsonianus ). Herring Gulls exhibit a well-documented suite of nest defense behaviors including vocalizations, aggressive displays and postures, and engagement in physical altercations with predators that vary with nest defense intensity ( Tinbergen, 1961 ). Herring Gulls and other closely related Larid species also demonstrate plasticity in nest defense behaviors under different cost-benefit scenarios, varying their intensity of defensive behavior in relation to perceived threat level from auditory ( MacLean & Bonter, 2013 ; Shah et al., 2015 ) and visual ( Covino et al., 2023 ) cues, and in response to offspring vulnerability ( Crisologo & Bonter, 2017 ). Herring Gulls have also been shown to observe diurnal activity patterns in nest attendance ( Morris, 1987 ) as well as individual and colony-level rest and activity patterns ( Galusha & Amlaner, 1978 ; Hayward et al., 2009 ), suggesting that factors impacting nest defense intensity may vary with time of day in this species. Our study addresses the gap in research on diurnal patterns in Herring Gull nest defense behavior. We experimentally simulated predatory threat at nests of incubating Herring Gulls at different times of day and quantified nest defense intensity. Previous work has shown that activity in Herring Gull colonies peaks in the early morning and late in the day ( Galusha & Amlaner, 1978 ), and studies of other Larid species suggest that agnostic behavior is positively correlated with colony activity ( Bukacińska & Bukaciński, 1994 ; Burger, 1976 , 1981 ; Conover & Miller, 1980 ). Thus, we predicted that nest defense would be most intense in the early morning and evening. Since whether one or both parents participated in nest defense can vary depending on whether the non-incubating parent is also present at the nest ( Crisologo & Bonter, 2017 ), we additionally investigated whether nest defense intensity varied with joint nest defense. Understanding such fine-scale variation in nest defense will provide more insight into the relative cost-benefits of nest defense. Such information can also help guide research on Herring Gulls and other colonially nesting species. Methods Our study site was Appledore Island, Maine (42.9891° N, 70.6142° W) which is home both to a mixed species colony of approximately 500 Herring Gulls and 300 Great Black-backed Gulls ( Larus marinus ), and to the Shoals Marine Laboratory, a summer undergraduate-focused research campus. Additionally, the Gulls of Appledore Research Group has been conducting research on the gull populations since at least 2004 ( Bonter et al., 2014 ; Covino et al., 2023 ; Ellis et al., 2007 ; MacLean & Bonter, 2013 ; Shah et al., 2015 ). We conducted our study on 26 Herring Gull nests, at all of which the parents were already incubating either two ( N =2) or three ( N =24) eggs. We selected focal nests that were relatively isolated from one another and that did not have a nearest neighbor within 10 meters to prevent a response from neighboring birds from interfering with the trial. To investigate diurnal variation in nest defense, we conducted trials during four time periods: “Early Morning” (0530-0600), “Late Morning” (1000-1030), “Afternoon” (1400-1430), and “Evening” (1900-1930). We conducted trials from 4–9 June 2024 when local sunrise and sunset times were approximately 0504 and 2019 respectively. Trials were not conducted in the event of rain to avoid interrupting incubation in inclement weather. In the 7 cases when eggs hatched prior to completing all trials, we ceased conducting trials on that nest since nest behaviors may vary across nesting stages ( Crisologo & Bonter 2017 ). Thus, our trial sample sizes for response score varied across the four time periods as follows: N =23 early morning, N =25 late morning, N =21 afternoon, N =21 evening. For each trial, a person, the “intruder”, walked toward the focal nest, stood 1 meter from the nest for 30 seconds, and then walked away from the nest. The intruder always walked at a steady pace with a neutral posture and expression while looking at the ground. Nest defense intensity was scored using a numerical scale ( Covino et al., 2023 ; MacLean & Bonter, 2013 ; Shah et al., 2015 ) as the most intense response that was exhibited by either the initially-incubating gull or the non-incubating mate (if present within observable distance to the nest) throughout the 30 second test period. A second person, the “observer”, positioned themselves far enough away from the focal nest where their presence did not disturb the gull and independently scored the nest defense intensity. After 30 seconds the intruder began walking away from the nest and the observer recorded the amount of time taken for the initially incubating gull to return to its previous state before the trial began ( i . e ., incubating without any defensive behavior). This measure of latency to calm was not recorded if the observer could not clearly see the gull return to the nest and was tracked for a maximum of 300 seconds (latency to calm sample sizes; N =22 early morning, N =24 late morning, N =20 afternoon, N =19 evening). Intruders wore standardized clothing (neutral-colored and long-sleeved) and a bicycle helmet (black or gray) to avoid adding confounding variables ( Covino et al., 2023 ). For each focal nest, the order in which the time-of-day trials were conducted was randomized and each focal nest experienced each of the four time of day trials only once. Additionally, the intruder was also randomized across all trials and 8 total intruders, such that no nest experienced the same intruder twice. To investigate the effect of time of day on nest defense intensity, we fit two linear mixed effect models with response score and latency to calm as dependent variables and time of day as the categorical fixed effect. We added date and nest ID as random effects since we conducted multiple trials at each nest and multiple trials per day. We used the “anova” function to run a type II ANOVA for each model. We also fit separate models for both dependent variables with mate presence (yes/no) as the fixed effect and only date as a random effect since mate presence did not vary randomly with nest ID ( Fig. 1 ). Finally, we ran a post-hoc chi-square test to examine if incubating gulls were less likely to get off the nest (yes/no) if their mate was present (yes/no) during the predation simulation trial. All analyses were performed in R version 4.2.2 ( R Core Team, 2022 ). Models were fit using the R package “lme4” ( Bates et al., 2015 ). Variance inflation factor < 2 for all fixed effects. Download figure Open in new tab Figure 1. Presence of the non-incubating mate was not randomly distributed among the trials conducted at our focal nests. Two nests, for example, consistently had the mate present for all four trials ( i . e ., proportion of trials per nest with mate present = 1.00), whereas most nests ( N = 11), never had the mate present in the vicinity for any of the trials ( i . e ., proportion of trials per nest with mate present = 0.00). Results Response scores of incubating Herring Gulls to a simulated predator threat ranged from 0 ( i . e ., no response or flees nest with no other reaction) to 8 ( i . e ., swooping and direct aerial attacks towards the intruder) with a mean (± SD) of 5.17 (± 2.18). Additionally, 86 out of 90 total trials (95%) had a response score of at least 1 ( Fig. 2A ). Latency to calm for the parent on the nest ranged from 0 seconds to our maximum measure of 300 seconds, with a mean (± SD) of 50.30 (± 82.13) seconds ( Fig. 2B ). Download figure Open in new tab Figure 2. Variation in (A) Response score ( N = 26 nests) (0: least to 8: most aggressive and energy intense response, see Table 1 for details), and (B) Latency to calm ( N = 25 nests), or time taken for the focal bird to return its pre-trial stage ( i . e ., incubating without any defensive behavior) of herring gulls at nests to a simulated predatory threat. Each nest (x-axis) experienced the same simulated predator—a human standing next to the nest for 30 seconds—up to 4 times total to test for an effect of time of day (“Early Morning”: 0530-0600, “Late Morning”: 1000-1030, “Afternoon”: 1400-1430, “Evening”: 1900-1930) across 6 days. Nests are arranged in order of median response score or latency to calm (lower to highest) for visual purposes. We found that Herring Gulls did not exhibit variation in nest defense intensity with time of day in terms of both response score ( F 3, 61.61 = 0.92, P = 0.43) or latency to calm ( F 3, 57.66 = 1.58, P = 0.20) ( Fig. 3 ). However, we found that nest defense intensity was significantly higher when the incubating gull’s mate also participated in nest defense in terms of response score ( F 1, 88 = 6.32, P = 0.01) ( Fig. 4A ), though not latency to calm ( F 1, 79.94 = 0.09, P = 0.77) ( Fig. 4B ). The presence of the mate did not affect the incubating gull’s likelihood of moving off the nest during the trial ( χ 2 = 1.95, df = 1, P = 0.16). View this table: View inline View popup Download powerpoint Table 1. Ethogram to rate the behavioral response of Herring Gulls. The response score increases with intensity of aggression and energy expenditure, modified from ( Covino et al. 2023 ). Download figure Open in new tab Figure 3. Herring Gulls at nests did not exhibit a variation in (A) Response score ( N = 26 nests, 90 trials) or (B) Latency to calm ( N = 25 nests, 85 trials) across four different times of day (x-axis). Response score was measured based on an ethogram of response aggressiveness and intensity to a potential predator in Herring Gulls ( Table 1 ), and latency to calm was measured as time in seconds taken for the focal bird to return its pre-trial stage ( i . e ., incubating without any defensive behavior), with a maximum cutoff at 300 seconds. Both behavioral measures were recorded in response to a simulated predatory threat of a human walking up to and standing for 30 seconds next to a Herring Gull incubating a nest. Download figure Open in new tab Figure 4. (A) The response score of Herring Gulls at a focal nest to a simulated predatory threat was significantly higher when the incubating individual’s mate was present in the vicinity of the nest than when the mate was absent (x-axis) ( N = 26 nests, 90 trials). However, (B) Latency to calm did not vary with mate presence ( N = 25 nests, 85 trials). Response score was measured based on an ethogram of response aggressiveness and intensity to a potential predator in Herring Gulls ( Table 1 ), and latency to calm was measured as time in seconds taken for the focal bird to return its pre-trial stage ( i . e ., incubating without any defensive behavior). Both behavioral measures were recorded in response to a simulated predatory threat of a human walking up to and standing for 30 seconds next to a herring gull incubating a nest. Discussion Since nest defense can prove energetically costly, birds often adjust the intensity of nest defense behavior based on various internal and external factors ( Montgomerie & Weatherhead, 1988 ). Such factors may vary with time of day ( Conover & Miller, 1980 ; Drent, 1970 ; Galusha & Amlaner, 1978 ; Kelly & Wood, 1996 ; Robbins, 1981 ), thus impacting the intensity of nest defense behavior. Here, we investigated whether nest defense in Herring Gulls, a colonial ground-nesting seabird with a suite of nest defense behaviors that have been shown to vary with perceived threat ( Covino et al., 2023 ; Crisologo & Bonter, 2017 ; MacLean & Bonter, 2013 ; Shah et al., 2015 ), varies with time of day. We found no difference in nest defense intensity between the early morning, late morning, afternoon, and evening, indicating that time of day does not impact the nest defense in Herring Gulls in our study population on Appledore Island, Maine. However, we found that nest defense intensity is higher when both parents are present at the nest. Overall, our results suggest that avoiding research-associated nest disturbance when both parents are present at the nest may reduce intensity of nest defense and minimize stress which can alter parental behavior and negatively impact reproductive success ( K. M. Brown & Morris, 1995 ; McNew et al., 2024 ). Nest defense has been shown to vary with time of day in some avian species including seabirds such as Black Skimmers ( Rynchops niger ) ( Burger, 1981 ) and Black-headed Gulls ( Larus ridibundus ) ( Bukacińska & Bukaciński, 1994 ), though not in a consistent manner. While nest defense in Black Skimmers peaked early in the morning ( Burger, 1981 ), Black-headed Gulls showed a peak in the late afternoon, the latter likely associated with increased movement of birds to and from foraging sites ( Bukacińska & Bukaciński, 1994 ). Contrary to both results, we found no variation in nest defense intensity with time of day in Herring Gulls. However, while previous studies were based on observations of conspecific and heterospecific interactions, we used an experimental approach with human presence as a simulated predatory threat. Additionally, our samples largely included nests in areas frequented by humans where Herring Gulls nest in a looser colony structure ( Savoca et al., 2011 ) to ensure that experimental trials would not be subject to interference from birds at neighboring nests. Habituation to close human presence could thus explain the lack of variation in nest defense intensity with time of day in response to simulated predator threat, though previous work in our study system found no evidence of habituation ( MacLean & Bonter, 2013 ). We found that nest defense intensity was higher when the non-incubating mate was also present and participated in nest defense. Previous studies have found a similar correlation between joint nest defense and nest defense intensity in Herring Gulls ( Crisologo & Bonter, 2017 ). Moreover, males have been shown to be more aggressive than females in joint nest defense against conspecifics. While this difference may be partly driven by mate guarding, presence of the mate may also allow for more intense nest defense by one parent while the other parent guards the nest ( Regelmann & Curio, 1986 ). While we were unable to determine sex of the parents in our study, such division of parental care duties by sex is prevalent in passerines, where females perform the majority of incubation and brooding ( Kendeigh, 1952 ), and has also been shown to some extent in non-passerine species with biparental care where both sexes incubate (Mute Swans ( Cygnus olor ): ( Włodarczyk & Minias, 2015 ); Killdeer ( Charadrius vociferous ): ( Brunton, 1990 ); Thick-billed Murres ( Uria lomvi ): ( Paredes et al., 2006 ); Razorbills ( Alca torda ): ( Paredes et al., 2006 )). However, though many studies have found coordinated territorial defense against conspecific intruders in various avian taxa, to our knowledge no study has examined whether coordinated nest defense against a predator enables division of tasks, freeing up one parent to engage in greater intensity of defensive behavior while the other parent guards the offspring ( Larsen, 1991 ). Alternatively, both parents may engage in more aggressive nest defense jointly than when defending the nest alone if the presence of their mate ameliorates the risk of nest failure due to injury or death to one parent ( Montgomerie & Weatherhead, 1988 ) or as a response to their mate’s “cooperative” behavior ( Chase, 1980 ). While we did not find that incubating gulls were more or less likely to get off the nest when engaging in joint nest defense with their non-incubating mate, a more nuanced study of nest defense behavior of each parent singly versus both parents jointly is needed to understand coordination of nest defense behavior in species with biparental care. Overall our results suggest that researchers should minimize disturbance at Herring Gull nests when both mates are present since disturbance that elicits more aggressive responses from parents may increase stress, impacting parental behavior, reproductive success, and offspring physiology ( McNew et al., 2024 ; Riou et al., 2010 ; Siller Wilks et al., 2024 ). More broadly, our study highlights the importance of identifying factors impacting nest defense intensity when designing field methodology for avian research to minimize stress to the study species and, in the case of Herring Gulls and other large species with highly aggressive nest defense, to the researchers. Data Availability Data and code are available on GitHub at https://github.com/shailee93/diural_effects_joint_nest_defense_herring_gulls Acknowledgements We thank the Shoals Marine Laboratory for giving us access to Appledore Island and its facilities and the staff and faculty for their support. We thank Dylan Titmuss for their help with our experimental design, Mary Elizabeth Everett and Sarah J. Courchesne of the Gulls of Appledore Research Group for project support, and Mitchell Norris for data collection in the field. Students in the 2024 Field Ornithology Class were supported by the Jackson Scholarship Fund, the John and Katharyn Williams Scholarship Fund, the Parson’s Scholarship Fund, the Rutmans Scholars Initiative, and the Spaulding Scholarship Fund. We also greatly appreciate an anonymous donor for their continued financial support of Gulls of Appledore Research Group and L. William Clark for his inspiration and financial support of various Appledore gull projects. SSS was supported by a National Science Foundation Postdoctoral Research Fellowship in Biology. KMC and the Loyola Marymount University Physiology, Hormones, & Avian Biology Lab are also supported by the Santa Monica Bay Audubon Society and by a Kadner-Pitts Research Grant. This is contribution #213 of the Shoals Marine Laboratory Footnotes https://github.com/shailee93/diural_effects_joint_nest_defense_herring_gulls References ↵ Bates , D. , Mächler , M. , Bolker , B. , & Walker , S. ( 2015 ). 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