The ecology and evolution of colony-size variation

Stay close, but not too close: aerial image analysis reveals patterns of social distancing in seal colonies

Many species aggregate in dense colonies. Species-specific spatial patterns provide clues about how colonies are shaped by various (a)biotic factors, including predation, temperature regulation or disease transmission. Using aerial imagery, we examined these patterns in colonies on land of two sympatric seal species: the harbour seal and grey seal. Results show that the density of grey seals on land is twice as high as that of harbour seals. Furthermore, the nearest neighbour distance (NND) of harbour seals (median = 1.06 m) is significantly larger than that of grey seals (median = 0.53 m). Avoidance at small distances (i.e. social distancing) was supported by spatial simulation: when the observed seal locations were shuffled slightly, the frequency of the smallest NNDs (0-25 cm) increased, while the most frequently observed NNDs decreased. As harbour seals are more prone to infectious diseases, we hypothesize that the larger NNDs might be a behavioural response to reduce pathogen transmission. The approach presented here can potentially be used as a practical tool to differentiate between harbour and grey seals in remote sensing applications, particularly in low to medium resolution imagery (e.g. satellite imagery), where morphological characteristics alone are insufficient to differentiate between species.

A population in perpetual motion: Highly dynamic roosting behavior of a tropical island endemic bat

Although island endemic bats are a source of considerable conservation concerns, their biology remains poorly known. Here, we studied the phenology and roosting behavior of a tropical island endemic species: the Reunion free-tailed bat (Mormopterus francoismoutoui). This widespread and abundant species occupies various natural and anthropogenic environments such as caves and buildings. We set up fine-scale monitoring of 19 roosts over 27 months in Reunion Island and analyzed roost size and composition, sexual and age-associated segregation of individuals, as well as the reproductive phenology and body condition of individuals. Based on extensive data collected from 6721 individuals, we revealed a highly dynamic roosting behavior, with marked seasonal sex-ratio variation, linked to distinct patterns of sexual aggregation among roosts. Despite the widespread presence of pregnant females all over the island, parturition was localized in a few roosts, and flying juveniles dispersed rapidly toward all studied roosts. Our data also suggested a 7-month delay between mating and pregnancy, highlighting a likely long interruption of the reproductive cycle in this tropical bat. Altogether, our results suggest a complex social organization in the Reunion free-tailed bat, with important sex-specific seasonal and spatial movements, including the possibility of altitudinal migration. Bat tracking and genetic studies would provide additional insights into the behavioral strategies that shape the biology of this enigmatic bat species. The fine-scale spatiotemporal data revealed by our study will serve to the delineation of effective conservation plans, especially in the context of growing urbanization and agriculture expansion in Reunion Island.

Climate fluctuations influence variation in group size in a cooperative bird

Variation in group size is ubiquitous in social animals, but explaining the range of group sizes seen in nature remains challenging.^1-3 Group-living species occur most frequently in climatically unpredictable environments, such that the costs and benefits of sociality may change from year to year.^4-6 It is, therefore, possible that variation in climate may help to maintain a range of group sizes, but this hypothesis is rarely tested empirically.^7,8 Here, we examine selection on breeding group size in the greater ani (Crotophaga major), a tropical bird that nests in cooperative groups containing multiple co-breeders and non-breeding helpers.⁹ We found that larger groups experience lower nest predation (due to cooperative nest defense) but suffer higher nestling starvation (due to intra-clutch competition). Long-term data revealed that the relative magnitude of these costs and benefits depends on climate, with frequent changes across years in the strength and direction of selection on group size. In wet years, individual reproductive success was higher in large groups than in small groups, whereas the opposite was true in dry years. This was partly a consequence of competition among nestlings in large clutches, which suffered significantly higher mortality in dry years than in wet years. Averaged over the 13-year study period, annual reproductive success was approximately equal for females in small and large groups. These results suggest that temporal changes in the direction of selection may help explain the persistence of a range of group sizes and that a full understanding of the selective pressures shaping sociality requires long-term fitness data.

Contrasting haemoparasite prevalence in larid species with divergent ecological niches and migration patterns

Haemoparasites represent a diverse group of vector-borne parasites that infect a wide range of vertebrate hosts. In birds, haemoparasite infection rates may be associated with various ecological and life history traits, including habitat choice, colony size and migration distance. Here, we molecularly assessed the prevalence of 3 main haemoparasite genera (Plasmodium, Haemoproteus and Leucocytozoon) in 2 bird species with different habitat preferences and migratory behaviour: black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo). We found that gulls showed a much higher prevalence and diversity of Plasmodium or Haemoproteus (ca. 60% of individuals infected) than terns (zero prevalence). The prevalence of Leucocytozoon was low in both species (<3%). The differences in haemoparasite prevalences may be primarily driven by varying vector encounter rate resulting from different habitat preferences, as black-headed gulls mainly use vector-rich vegetated freshwater habitats, whereas common terns often use vector-poor coastal and brackish habitats. Since common terns migrate further than black-headed gulls, our results did not provide support for an association between haemoparasite prevalence and migratory distance. In gulls, we found a negative association between colony size and infection rates, suggestive of an ideal despotic distribution, and phylogenetic analyses of detected haemoparasite lineages provided evidence for higher host specificity in Haemoproteus than Plasmodium. Our results suggest that the preference for coastal areas and less vegetated habitats in terns may reduce haemoparasite infection rates compared to other larids, regardless of their migratory distance, emphasizing the role of ecological niches in parasite exposure.

Foraging range scales with colony size in high-latitude seabirds

Density-dependent prey depletion around breeding colonies has long been considered an important factor controlling the population dynamics of colonial animals.^1-4 Ashmole proposed that as seabird colony size increases, intraspecific competition leads to declines in reproductive success, as breeding adults must spend more time and energy to find prey farther from the colony.¹ Seabird colony size often varies over several orders of magnitude within the same species and can include millions of individuals per colony.^5,6 As such, colony size likely plays an important role in determining the individual behavior of its members and how the colony interacts with the surrounding environment.⁶ Using tracking data from murres (Uria spp.), the world's most densely breeding seabirds, we show that the distribution of foraging-trip distances scales to colony size^0.33 during the chick-rearing stage, consistent with Ashmole's halo theory.^1,2 This pattern occurred across colonies varying in size over three orders of magnitude and distributed throughout the North Atlantic region. The strong relationship between colony size and foraging range means that the foraging areas of some colonial species can be estimated from colony sizes, which is more practical to measure over a large geographic scale. Two-thirds of the North Atlantic murre population breed at the 16 largest colonies; by extrapolating the predicted foraging ranges to sites without tracking data, we show that only two of these large colonies have significant coverage as marine protected areas. Our results are an important example of how theoretical models, in this case, Ashmole's version of central-place-foraging theory, can be applied to inform conservation and management in colonial breeding species.

Foraging Payoffs Change With Group Size in Kin and Non-kin Groups of an Argyrodinae Kleptoparasitic Spider, Argyrodes miniaceus

Evolutionary transitions from solitary to group-living are ubiquitous in animal systems. While the fitness consequences of group size changes are often investigated, the long-standing debate on whether kinship is a prerequisite of sociality is still ongoing. In the current study, we used kleptoparasitic spiders Argyrodes miniaceus (subfamily Argyrodinae, Theridiidae) as a model system to assess the role of group size on the foraging payoffs of kin and non-kin groups. We set up laboratory-manipulated kin and non-kin foraging groups and used feeding occurrence and duration as proxies for foraging benefits and feeding latency and the number of host attacks as estimates of foraging costs. Compared to solitary individuals, feeding durations of successfully fed individuals in groups was not significantly different from that of solitary foragers in both kin and non-kin groups. The occurrences of feeding decreased significantly in group sizes two and above, in non-kin groups, and in group sizes three and above, in kin groups. In kin groups, groups size two had significantly shorter feeding latencies compared to other group sizes, even though feeding duration did not change systematically with group size. Similarly, the number of attacks from the hosts were highest in non-kin groups with more than two individuals and in kin groups with more than three individuals. The juxtaposition of kin and non-kin group showed that A. miniaceus enjoyed the highest foraging payoffs when being solitary or in small groups (group size two). However, host attacks appeared to hamper feeding occurrences in kin groups, which was not observed in non-kin groups. Our results contrast sharply with the feeding benefits of kinship recorded in kin-based groups of sub-social species present in related subfamilies in the Theridiidae.

Birds of a Feather Flock Together: Extent of Long-Term Consistency of Colony-Size Choice in Cliff Swallows

Explaining why animal groups vary in size is a fundamental problem in behavioral ecology. One hypothesis is that life-history differences among individuals lead to sorting of phenotypes into groups of different sizes where each individual does best. This hypothesis predicts that individuals should be relatively consistent in their use of particular group sizes across time. Little is known about whether animals’ choice of group size is repeatable across their lives, especially in long-lived species. We studied consistency in choice of breeding-colony size in colonially nesting cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, United States, over a 32-year period, following 6,296 birds for at least four breeding seasons. Formal repeatability of size choice for the population was about 0.41. About 45% of individuals were relatively consistent in choice of colony size, while about 40% varied widely in the colony size they occupied. Birds using the smaller and larger colonies appeared more consistent in size use than birds occupying more intermediate sized colonies. Consistency in colony size was also influenced by whether a bird used the same physical colony site each year and whether the site had been fumigated to remove ectoparasites. The difference between the final and initial colony sizes for an individual, a measure of the net change in its colony size over its life, did not significantly depart from 0 for the dataset as a whole. However, different year-cohorts did show significant net change in colony size, both positive and negative, that may have reflected fluctuating selection on colony size among years based on climatic conditions. The results support phenotypic sorting as an explanation for group size variation, although cliff swallows also likely use past experience at a given site and the extent of ectoparasitism to select breeding colonies.

Change in beak overhangs of cliff swallows over 40 years: Partly a response to parasites?

Some birds exhibit a maxillary overhang, in which the tip of the upper beak projects beyond the lower mandible and may curve downward. The overhang is thought to help control ectoparasites on the feathers. Little is known about the extent to which the maxillary overhang varies spatially or temporally within populations of the same species. The colonial cliff swallow (Petrochelidon pyrrhonota) has relatively recently shifted to almost exclusive use of artificial structures such as bridges and highway culverts for nesting and consequently has been exposed to higher levels of parasitism than on its ancestral cliff nesting sites. We examined whether increased ectoparasitism may have favored recent changes in the extent of the maxillary overhang. Using a specimen collection of cliff swallows from western Nebraska, USA, spanning 40 years and field data on live birds, we found that the extent of the maxillary overhang increased across years in a nonlinear way, peaking in the late 2000's, and varied inversely with cliff swallow colony size for unknown reasons. The number of fleas on nestling cliff swallows declined in general over this period. Those birds with perceptible overhangs had fewer swallow bugs on the outside of their nest, but they did not have higher nesting success than birds with no overhangs. The intraspecific variation in the maxillary overhang in cliff swallows was partly consistent with it having a functional role in combatting ectoparasites. The temporal increase in the extent of the overhang may be a response by cliff swallows to their relatively recent increased exposure to parasitism. Our results demonstrate that this avian morphological trait can change rapidly over time.

Variation in male-built nest volume with nesting-support quality, colony, and egg production in whiskered terns

Nest building can represent an energetically costly activity for a variety of animal taxa. Besides, the determinants of within-species variation in the design of nests, notably with respect to natural and sexual selection, are still insufficiently documented. Based on an observational study, we examined the influence of nesting conditions (nesting-support quality, colony, laying date, and year) on male-built nest volume and also its potential role as a postmating sexually selected display in the whiskered tern Chlidonias hybrida. This tern species is a monogamous colonial bird with obligate biparental care breeding on aquatic vegetation. Hence, large nesting platforms are expected to be a selective advantage because they would better withstand adverse environmental conditions and provide a secure structure for eggs. Nest size may also serve as a postmating sexual trait, and variation in egg production would be positively associated with nest size. We found that nest volume was adjusted to different environmental cues. A positive relationship was found between nest volume and nesting-support quality, indicating that the leaf density of white waterlily is essential for nest stability. Variation in nest volume was not correlated to colony size but varied among colonies and years. Male-built nest volume was also positively associated with mean egg volume per clutch but not with clutch size. The fitness consequences of building a large nest are yet to be studied, and additional investigations are recommended to better understand whether the activity of males early during breeding season (e.g., nest building and courtship feeding performance) really serves as postmating sexually selected signals.

Heronry distribution and site preference dynamics of tree-nesting colonial waterbirds in Tamil Nadu

Anthropogenic disturbance and climate change have significantly affected the distribution of wetlands globally and particularly in Asian countries. Various types of wetlands are harboured across all the biogeographic zones in India. These wetlands provide vital ecological services and are rich in biodiversity. However, anthropogenic pressures continue to be a threat to these wetlands by affecting the flora and fauna that depend on them. Tree-nesting colonial waterbirds are vulnerable to these pressures as their colonies are typically located in wetlands and associated areas. Disturbances to these areas have resulted in the loss or shifting of many heronries. The present study was conducted in the Indian state of Tamil Nadu during the period of 2017–2019 to document the existing and previously unknown heronries of the landscape. A total of 101 heronries were documented in 22 districts. The Little Cormorant was the most dominant species, occurring in 79% of the sites, with relative abundances of 24% and 26% during 2017–2018 and 2018–2019, respectively. A total of 23 tree species were utilized by the birds for nesting and Vachellia nilotica trees were used for nesting in about 25% of the heronries. 19% of the heronries were situated inside protected areas and 81% were located outside protected areas. Out of the 58 active nesting sites reported in 2005, 43 have been lost or are no longer active. Species distribution modelling with presence only data indicated that the sites with a high probability of occurrence were confined to the major waterbodies and rivers. Spatial correlation showed that the heronries were dispersed randomly across the landscape. The population dynamics within heronries and colonial nesting waterbirds’ response to various environmental factors must be monitored continuously to conserve these heronries.

The influence of subcolony-scale nesting habitat on the reproductive success of Adélie penguins

Group-size variation is common in colonially breeding species, including seabirds, whose breeding colonies can vary in size by several orders of magnitude. Seabirds are some of the most threatened marine taxa and understanding the drivers of colony size variation is more important than ever. Reproductive success is an important demographic parameter that can impact colony size, and it varies in association with a number of factors, including nesting habitat quality. Within colonies, seabirds often aggregate into distinct groups or subcolonies that may vary in quality. We used data from two colonies of Adélie penguins 73 km apart on Ross Island, Antarctica, one large and one small to investigate (1) How subcolony habitat characteristics influence reproductive success and (2) How these relationships differ at a small (Cape Royds) and large (Cape Crozier) colony with different terrain characteristics. Subcolonies were characterized using terrain attributes (elevation, slope aspect, slope steepness, wind shelter, flow accumulation), as well group characteristics (area/size, perimeter-to-area ratio, and proximity to nest predators). Reproductive success was higher and less variable at the larger colony while subcolony characteristics explained more of the variance in reproductive success at the small colony. The most important variable influencing subcolony quality at both colonies was perimeter-to-area ratio, likely reflecting the importance of nest predation by south polar skuas along subcolony edges. The small colony contained a higher proportion of edge nests thus higher potential impact from skua nest predation. Stochastic environmental events may facilitate smaller colonies becoming “trapped” by nest predation: a rapid decline in the number of breeding individuals may increase the proportion of edge nests, leading to higher relative nest predation and hindering population recovery. Several terrain covariates were retained in the final models but which variables, the shapes of the relationships, and importance varied between colonies.

Colony size as a predictor of breeding behaviour in a common waterbird

The choice of colony size may have profound consequences for individual fitness in colonially breeding birds, but at the same time it may require certain behavioural adaptations. Here, we aimed to examine behavioural divergence of common terns Sterna hirundo nesting in colonies of different size. For this purpose, we promoted establishment of small (<35 pairs) and large (>100 pairs) tern colonies under uniform ecological and environmental conditions by providing attractive patches of nesting substrate (floating rafts) at a single site. We combined video recording and GPS-tracking to assess communal and individual defence initiation rate, intra-specific aggression rate, and foraging flight characteristics. We found that birds from larger colonies more frequently engaged in communal defence and they performed longer foraging flights, while terns from smaller colonies more frequently showed individual defence behaviours. Also, intra-specific aggression rate was higher in smaller colonies, but this effect was primarily attributed to a higher proportion of edge breeding pairs, which were more aggressive. Our results suggest that various colony sizes may be associated with different behavioural syndromes, which comprise of diverse personality traits, such as social responsiveness, social tolerance, or propensity for aggression. It remains to be tested whether these behavioural differences reflect processes of phenotypic sorting among colonies of different size or whether they are a result of behavioural plasticity under different social contexts.

Decrease in social cohesion in a colonial seabird under a perturbation regime

Social interactions, through influence on behavioural processes, can play an important role in populations’ resilience (i.e. ability to cope with perturbations). However little is known about the effects of perturbations on the strength of social cohesion in wild populations. Long-term associations between individuals may reflect the existence of social cohesion for seizing the evolutionary advantages of social living. We explore the existence of social cohesion and its dynamics under perturbations by analysing long-term social associations, in a colonial seabird, the Audouin’s gull Larus audouinii, living in a site experiencing a shift to a perturbed regime. Our goals were namely (1) to uncover the occurrence of long-term social ties (i.e. associations) between individuals and (2) to examine whether the perturbation regime affected this form of social cohesion. We analysed a dataset of more than 3500 individuals from 25 years of monitoring by means of contingency tables and within the Social Network Analysis framework. We showed that associations between individuals are not only due to philopatry or random gregariousness but that there are social ties between individuals over the years. Furthermore, social cohesion decreased under the perturbation regime. We sustain that perturbations may lead not only to changes in individuals’ behaviour and fitness but also to a change in populations’ social cohesion. The consequences of decreasing social cohesion are still not well understood, but they can be critical for the population dynamics of social species.

Ecological correlates of group integrity among dispersing cliff swallows

Breeding colonies of birds represent groups of individuals that associate during one breeding season, at least partially dissociate for the non-breeding season, and may re-associate the next year through collective settlement at another breeding site. Little is known about the extent to which colonial birds maintain group integrity when occupying different sites in different years or the benefits of long-term associations among colonial individuals. For cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, USA, we examined ecological correlates and potential benefits associated with group integrity. Using a dataset of over 25,000 individuals, we found that associations between dispersing cliff swallows were greater than would be predicted by purely random settlement among colony sites. The extent of group integrity varied widely among sites, with birds seeming to settle together more often at sites with fewer ectoparasites and at colonies similar in size and closer in physical proximity to the one they had previously occupied. Some associations of birds lasted three years in which they used a different site each year. Successful colonies had higher levels of group integrity among their settlers than did colonies that failed completely. Cliff swallows that were known to have settled with at least one conspecific from the previous year's site had a higher survival the next year than those not known to have settled with past residents. The results are consistent with cliff swallows choosing colonies based in part on parasite load and with sorting among colonies based on the birds' preferences for colonies of certain sizes.

Variation in neophobia among cliff swallows at different colonies

Animal groups often represent nonrandom subsets of individuals, and increasing evidence indicates that individuals may sort among groups based on their personalities. The size of a group can predict its personality composition in some species due to differential suitability of a personality for groups of certain sizes, and the group itself may function more effectively if particular personality types are present. We quantified cliff swallow (Petrochelidon pyrrhonota) behavioral measures using linear and generalized linear mixed models to identify whether they: (1) varied among individuals within colonies and among colonies, (2) were related to reproductive success, and (3) predicted levels of parental care. Significant among-individual and among-colony site variation in a cliff swallow's latency to enter its nest when presented with a novel stimulus was revealed. We also found significant among-individual variation in the number of attacks directed toward a novel stimulus at the nest and in the response to broadcast of a cliff swallow alarm call recording, but among site variation in these measures was not significant. We did not find evidence for behavioral syndromes linking the personalities measured. Differences among individuals in latency to enter the nest and the number of attacks were not significantly related to reproductive success or to the extent to which birds fed their nestlings. However, extent of nestling feeding was significantly predicted by the number of mist net captures. The limited evidence in general of systematic variation in the behavior we measured among cliff swallow colonies may reflect the different and sometimes opposing selection pressures on behavior in different social environments. Future work should perhaps examine variation in other behavioral traits, such as foraging, in cliff swallow colonies of different sizes.

Colony size affects nestling immune function: a cross-fostering experiment in a colonial waterbird

Elevated transmission rate of pathogens and parasites is considered one of the major costs of sociality in birds. However, greater risk of infection in colonial birds might be compensated by specific immune adaptations. Here, we predicted that nestlings raised in larger colonies should invest more in their immune function. To test this hypothesis, we manipulated colony size and conduced cross-fostering experiment in a colonial waterbird, the common tern Sterna hirundo. Establishment of different size colonies under uniform environmental conditions was induced by providing large and small patches of attractive nesting area for terns (floating rafts). Then, pairs of clutches were swapped between large and small tern colonies, and skin-swelling response to phytohaemagglutinin (PHA) was assessed for nestlings from experimental and control broods. Contrary to our expectations, we found a negative effect of foster colony size on nestling PHA response (nestlings raised in the larger colony had lower PHA response). In addition, nestling PHA response correlated negatively with heterophil/lymphocyte ratio used as a measure of physiological stress. This suggested that low PHA response of nestlings raised in the larger colony could be mediated by an elevated level of social stress. We suggest that depression of immune function via social stress may constitute a strong selective pressure against large colony size in the common tern, and possibly in other colonial species. We also recommend that this largely overlooked cost of sociality should be considered in the further studies on the evolution and ecology of avian coloniality.

Spatio-temporal variation in nesting success of colonial waterbirds under the impact of a non-native invasive predator

Colonial breeding in birds provides protection from predators and may be particularly important when birds have to cope with an invasive predator. The probability of nest predation in a colony can vary depending on several factors, such as the nest’s location in the colony and the level of aggregation of nests. We studied the nesting success of colonial great crested grebes and monitored the occurrence of the non-native invasive American mink in the colony. From among 92 grebe nests, 54.3% were successful. The daily survival rate (DSR) of grebe nests was positively affected by the increasing distance between the nest and lake shoreline, and negatively affected by the increasing distance between the nest and the five nearest grebe nests. The probability of mink occurrence in the colony increased with consecutive days of the breeding season and decreased with increasing distance from the lake shoreline. The DSR of grebe nests decreased with the increasing probability of mink occurrence along the shoreline distance gradient and the day of the breeding season. The results of the study confirm the impact of the American mink on waterbirds during the breeding season but also indicate that large breeding colonies are partially safe from mink predation, and that nest accessibility and the dilution effect influence the probability of nest survival. Our data suggest that the limited access to safe breeding sites on large lakes that can supply adult grebes and their chicks with food may affect bird productivity and population numbers at the landscape level.

Why come back home? Breeding-site fidelity varies with group size and parasite load in a colonial bird

Fidelity to a past breeding site is widespread among animals and may confer both costs and benefits. Colonial species occur at specific sites that can accommodate multiple breeders, and the choice of whether to return to last year's site or disperse elsewhere can affect colony site use, the colony size distribution and individual fitness. For the colonial cliff swallow, Petrochelidon pyrrhonota, which occupies colonies of widely different sizes, we used a 30-year field study in western Nebraska to investigate how the extent of infestation by ectoparasites and colony size affected breeders' colony site fidelity between years. We compared philopatry at colonies where parasitic swallow bugs, Oeciacus vicarius, had been removed by fumigation with that at nonfumigated sites exposed to natural levels of ectoparasites. About 25% of birds at nonfumigated colonies returned to their previous year's site, whereas about 69% of birds at fumigated colonies did so. Site fidelity was greatest at nonfumigated sites that changed the least in size between years. Birds were less likely to return to a nonfumigated site as the colony there became increasingly larger. Individuals philopatric to both nonfumigated and fumigated sites resided in colonies more similar in size between years than did dispersing birds. Most cliff swallows settled within 6 km of their previous year's site, indicating that many nonphilopatric birds still may have had some familiarity with the local landscape surrounding the site to which they moved. Removal of ectoparasites at a site allows large colonies to persist there perennially, probably contributing to higher philopatry because such large colonies are rare and would have been difficult to find had the residents dispersed. Cliff swallows are likely to be sensitive to both colony size and general familiarity with a given site or landscape region, and probably integrate these with other cues to select breeding colonies.