Density-Dependent and Predator-Specific Nest Defense Strategies in Colonially Breeding Saunders’s Gulls

Cooperative breeding and anti-predator strategies of the azure-winged magpie (Cyanopica cyanus Pallas, 1776) in northern Mongolia

Heterospecific disturbance in seabird colonies can negatively influence reproductive success both through direct effects, such as predation, and through indirect effects, such as increasing parental energy expenditure via defense behaviors. Here, remote nest cameras were used to evaluate the effects of intrusion on early nest survival and predation risk in Roseate Tern (Sterna dougallii) colonies in the U.S. and British Virgin Islands. Effects of colony traits such as colony size on parental nest defense were also investigated to assess individual response to predator presence in colonies. We counted 141 intrusion events at 118 nests and found that frequency of heterospecific disturbance did not influence nest survival, but did have a positive association with predation rate. Disturbance decreased significantly with colony size and % cover, and disturbance increased with nest density, indicating that concealed, isolated nests within large colonies were less likely to be disturbed. Parental nest defense decreased significantly with colony size and nest density. Results were likely driven by the dominant predator types in our system—large predatory birds and invasive rats—as parents were more likely to leave nests with intrusion from these predator types.

Nesting birds face significant risks of predation, prompting parents to invest in nest defense. However, not all environments are the same, and parental investment may vary across different environments. Urbanization often leads to habitat changes and an increase in generalist nest predators, potentially driving a higher risk of reproductive failure for birds. This may require urban-breeding birds to invest more time and energy in nest defense, in addition to balancing other essential activities, such as incubation, foraging, and feeding their young. Here, we assess the impact of urbanization on nest defense by white-winged choughs (Corcorax melanorhamphos). We predicted that (1) urban habitats have a greater abundance of nest predators compared to natural habitats, and that (2) nest defense would correspondingly be higher in urban-breeding white-winged choughs. We also predicted that (3) nest defense in the white-winged choughs would increase with the age of the brood, in accordance with the nest defense theory, irrespective of their habitat. Our results confirm a significantly greater abundance of avian nest predators in urban areas, and that nest defense is significantly higher in urban groups compared to groups in natural habitats. However, we found no significant increase in nest defense with the age of the brood. Our study confirms that the increase in nest predator abundance can cause urban birds to face different trade-offs, and have to invest more in nest defense. This reflects one effect of urbanization on the composition and behavior of urban wildlife.

Interindividual differences in nest defense towards a single predator type have been shown to be repeatable in multiple species, supporting the presence of personality. Here, we assessed if the nest defense of female northern house wrens (Troglodytes aedon) was repeatable across different types of predators, which is predicted by personality and largely untested. Over three years, we placed a decoy of a common nest predator, the eastern rat snake (Pantherophis alleghaniensis), on top of nest boxes and measured female behavior. Each season, we also presented a second stimulus of varying threat level, including an eastern chipmunk decoy (Tamias striatus), a taxidermied Cooper’s hawk (Accipiter cooperii), and a novel object of unknown threat. We measured repeatability between each pair of threats and compared the population-level response to the snake across years. Nest defense was significantly repeatable between the snake and chipmunk, which presented similar risks. It was also repeatable between the snake and novel object despite the population-level response to the object being significantly weaker. In contrast, nest defense was not repeatable between the snake and the hawk, which posed a significant threat to adult survival that may have disrupted the consistency of the female response. Finally, the average, population-level response to the snake did not detectably differ among years, indicating stability in this behavior despite high turnover in breeding adults. These results suggest the presence of personality in nest defense, but more research is needed to evaluate the effect of high-risk predators on the repeatability of this behavior.Significance StatementDifferences among individuals in the strength of nest defense are often repeatable towards a specific predator, indicating the presence of personality. However, few studies have tested the repeatability of nest defense across different predators of varying threat levels. Personality predicts that weak/strong responders to one predator should also be weak/strong to another. We tested this prediction in female northern house wrens by placing decoys of different predators and a novel object on top of their nest boxes to measure nest defense. Nest defense was repeatable between two predators of similar risk and between a predator and a novel object of unknown risk. However, nest defense was not repeatable between a low and high-risk predator. The results suggest the presence of personality, but research is needed on the effects of high-risk predators that appear to disrupt behavioral consistency.

The pattern of increased nest defense effort over the course of a nesting season could result from three distinct (albeit non-exclusive) mechanisms: increased value of offspring to parents with progression toward independence (parental investment theory), decreased opportunity for renesting (renesting potential hypothesis), or decreased perceived costs of defense after repeated encounters with human observers (positive reinforcement hypothesis). To gauge relative empirical support for each of these mechanisms, we disentangle these three often-confounded hypotheses using multimodel inference with mixed-model ordinal regression applied to an extensive red-winged blackbird (Agelaius phoeniceus) nesting data set (4518 monitoring visits to 1330 nests). Parent aggression was rated on an ordinal scale (0–4) during repeated monitoring visits. Additionally, we assessed clutch/brood size, nest density, time of day, and nest concealment effects on aggression. In a preliminary analysis, including all three major hypotheses, male and female nest defense was most strongly explained by parental investment (nest age). Positive reinforcement (visit number) and renesting potential (Julian date) were also well-supported predictors in males. The interactions of decomposed nest age (within-individual and between-individual centered) with Julian date were particularly important in the top male model. Additional factors, such as clutch/brood size, nest density, and nest concealment, appeared to have larger predictive roles in explaining female aggression relative to males. These patterns are likely explained by different sexual reproductive roles within a polygynous mating system. Our study highlights the importance of interacting mechanisms involving parental investment theory and the use of within-individual standardization to help disentangle competing and empirically confounded hypotheses. Avian nest defense generally increases over the course of a nesting season, potentially from the result of three different mechanisms: parental investment theory, renesting potential hypothesis, or positive reinforcement hypothesis from repeated nest visitation. We revisit this classic question through a comprehensive analytical approach with an extensive observational data set with red-winged blackbirds, employing multimodel selection and within-individual and between-individual centering techniques. We found that parental investment (nest age) was the strongest predictor of nest defense for both sexes; however, positive reinforcement and renesting potential also appeared to help explain additional variation in nest defense for males. Competitiveness of models with interactive effects indicated that these mechanisms do not operate independently for either sex, and additional covariates (e.g., clutch/brood size) especially aided female model competiveness. Our study highlights the importance of multiple and often interacting factors that influence avian nest defense.

Successful reproduction depends on the ability of parents to defend from nest predators. Breeding birds often put their life at risk to defend their offspring from predators. Communal nest defense has been proposed as an individual benefit of group living. We tested this by experimentally exposing a short-lived avian species to a potential risk of predation. We presented decoys of both a diurnal and a nocturnal predator to barn swallows Hirundo rustica breeding solitarily and in large colonies in South-western Spain. We found clear benefits of group living compared to solitary pairs in relation to investment in individual nest defense and the capacity to deter a challenging predator. Although we did not find differences in the time needed to detect the predator, we found that the number of individuals recruited to participate in nest defense was greater for colonial breeding pairs, leading to fewer attacks per individual in comparison to solitary breeding pairs. We also found that barn swallows defended their nests more vigorously against a nocturnal predator than against a diurnal predator. Therefore, barn swallows living in groups obtained clear benefits in communal nest defense by reducing the risk, energy and time dedicated to nest defense. Colonial breeding is thus a more effective strategy for decreasing nest predation rates.

The raccoon dog is a medium sized canid native to East-Asia. It was introduced to the western Soviet Union during the first half of the twentieth century, and has since then spread to, and established in, many European countries where it now is considered invasive. Raccoon dogs are suspected to have negative impacts on biodiversity, for example through nest predation, but empirical evidence is scarce. In this study we used GPS monitoring combined with camera traps on both artificial and natural nests to find out: (1) if raccoon dogs find and scavenge eggs from artificial nests, (2) if the scavenging from raccoon dogs is additive or compensatory to the scavenging from native species, and, (3) if raccoon dogs actively scare brooding birds off their nests and prey on their eggs. We found that raccoon dogs effectively located artificial nests and scavenged their eggs. There was a significantly higher scavenging frequency on experiment islands with both raccoon dogs and native scavengers, than on control islands with only native scavengers. There was no difference in native scavenging frequency on islands with versus without a raccoon dog, suggesting an additive effect from the raccoon dog on top of the native scavenging. GPS-tracked raccoon dogs moved intensively in the archipelago during the bird breeding season, swimming long distances to reach new islands if needed. Raccoon dogs that arrived on islands with natural nests actively scared brooding hens, up to the size of graylag goose, off their nests and preyed on their eggs. Raccoon dogs preyed on all the eggs they found, but discarded the egg shells. Not consuming the egg shells consequently leads to few visible traces of eggs in their stomachs or faeces, which in turn may explain why egg predation by raccoon dogs has been largely overlooked in previous studies. We discuss the potential impact of raccoon dogs on biodiversity, in the light of our new findings, and conclude that the raccoon dog may have a much larger effect on the breeding success of ground nesting sea birds than what has so far been the predominating view in the scientific literature.

Studies of avian nest defense generally explain only a small proportion of the total variation in defense behavior. We explored two potential methodological sources of variation in nest defense in three species of ducks. One common method of quantifying nest defense rests on the assumption that different components of nest defense (e.g. flushing distance, distraction displays) are highly positively correlated. Defense behaviors we observed in this study were weakly related or unrelated to each other. Thus, the assumption of strong positive covariance between components of nest defense was not supported. We also considered the effect of repeated visits to the same nests on nest defense. Females of all three species took less risk defending their nests with repeated visits, and the effect of visit number on nest defense was greater than the effect of increasing value of nests associated with advancing incubation. Ducks appear to be different from other birds in the consistency with which they alter their nest defense in response to repeated nest visits. We propose that this could be a consequence of having nest predators that return to the vicinity of a nest if they were previously unsuccessful finding the nest, thus making repeated nest visits more dangerous to the ducks. By testing this or other hypotheses it should be possible to go beyond understanding the methodological implications of the effect of repeated visits on nest defense, and use this phenomenon to gain insight into the predator‐prey interactions that underlie nest defense.

Offspring predation is one of the greatest obstacles to an organism's reproductive success, but parents vary in the strength of their response to potential predators. One explanation for this variable investment is that defending current offspring has the potential to lower future reproductive success if the predator is also capable of injuring or killing the parent. Northern house wrens (Troglodytes aedon) are cavity-nesting songbirds that defend against multiple species of nest predators including small mammals, birds of prey, and snakes. Here, we used three different predator decoys: two nest predators-an eastern chipmunk (Tamias striatus) and an eastern ratsnake (Pantherophis alleghaniensis)-as well as a predator of both offspring and adults-a juvenile Cooper's hawk (Accipiter cooperi)-to elicit nest defense and test whether females use risk assessment to modulate their antipredator behavior. We found that antipredator behaviors were not significantly different between the two nest predators, which posed a high risk to the nestlings, but lower risk to the parents, as neither species frequently captures adult wrens outside the nest box. However, female wrens never dove at or attacked the Cooper's hawk, while they frequently attacked both the snake and chipmunk decoys. Neighboring house wrens from adjacent territories were also less likely to respond to the hawk, but more heterospecifics mobbed the hawk than the snake decoy. Collectively, these results show that risk assessment and the strength of the antipredator response varies substantially both within and among species. Female house wrens exhibit plasticity in their nest defense behavior, and they respond to different types of predators in a way that could maximize lifetime fitness while risking the loss of their current offspring.

A large part of the variation in bird reproductive success is often explained by nest predation. Many studies report negative relationships between breeding density and reproductive success due to the predation effect. In this study, we tested the hypothesis that the total nest density should affect nest predation stronger than the nest density of a single species. We used a large sample (n = 320) of Booted Warbler Iduna caligata and Whinchat Saxicola rubetra nests obtained during a period of 6 years in abandoned fields. We used model selection to evaluate effects of density, nest age, season and year on daily nest survival rate. We used a single-species (the distance to the nearest conspecific nest and the number of conspecific nests around the focal nest) and total (the distance to the nearest nest of any passerine species and the total number of passerine nests around the focal nest) nest-density variables. Our results suggest that nest density affects nest survival negatively. Both Booted Warbler and Whinchat nests were more likely to be depredated when neighboring nests of any passerine species were closer. Daily nest survival rates were better predicted by the total nest density than single-species nest density. We suggest that generalist predators performing an area-restricted search may play an important role in nest predation in abandoned fields. The total nest density should be estimated when studying density-dependent nest predation as conclusions about nest predation based on single-species nest densities may be incorrect. The potential impact of density-dependent predation on real nests should be considered when designing artificial nest experiments.

Edge effects on grassland-nesting birds should be less pronounced or absent near cropland edges of grasslands that lack wooded-edge habitat often used by nest predators and brood parasites. We compared nest predation, brood parasitism and densities of dickcissel (Spiza americana) nests in relation to distance from woodland and cropland edges of Kansas tallgrass prairie. Daily nest predation rates did not differ (P > 0.25) among distance intervals (≤50 m, 51–100 m, ≤100 m and >100 m) from either edge type or among 50-m intervals adjacent to each edge type. Brood parasitism rates by the brown-headed cowbird (Molothrus ater) were higher ≤100 m vs. >100 m from woodland edges (P = 0.04), being highest ≤50 m from woodland edges (P = 0.09). Parasitism rates were not related to distance from cropland edges, although parasitism rates ≤50 m from woodland and cropland edges were statistically similar (P = 0.16). Dickcissel nest densities were lower ≤50 m from woodland edges relative to farther distance intervals (P = 0.004), indicating dickcissel avoidance of this edge type. There was no similar pattern of nest density on cropland-edged sites, but nest densities ≤50 m from woodland and cropland edges were statistically similar (P = 0.17). Thus, some woodland edge effects on this grassland bird species were apparent but might vary geographically.

Effects on female reproductive success of familiarity and experience among male red-winged blackbirds

Landscape change throughout North America has resulted in heightened nest predator population and declining avian productivity. Essential to establishing effective management design is an understanding of differential predation pressure among avian groups as group specific responses to predation impact may exist. The objective of this study was to examine the efficacy of predator trapping on the nest success and density of ground nesting avifauna in 2004-2005 in the Virginia Coast Reserve, specifically dabbling ducks, Canada Goose and Willet. Second, we determine the impact of predation on ground nesting birds by relating indices of predator abundance to nest density and nest success for island plots. Overall Mayfield nest success for dabbling ducks was 54.4% (n = 12) in 2004 and 17.7% (n = 30) in 2005. Green Transformed nest success for dabbling ducks was 34.5% (n = 25) in 2004 and 23.0% (n = 42). For Canada goose, overall Mayfield nest success was 53.1 (n = 37) in 2004 and 47.7% (n = 39) in 2005. Overall Green Transformed nest success for Canada Goose was 59.5% (n = 57) in 2004 and 50.6% (n = 51) in 2005. Finally, overall Green Transformed nest success for Willet was 53.7% (n = 110) in 2004 and 46.0% (n = 118) in 2005. Nest success estimates on island plots varied greatly. There was no difference in nest success between trapped and non-trapped islands for dabbling ducks (P = 0.1990), Canada Goose (P = 0.4860), Willet (P = 0.4920) and artificial nest success (P = 0.4200). Likewise, there was no difference in nest density between trapped and non-trapped islands for dabbling ducks (P = 0.2408), Canada Goose (P = 0.2950), and Willet (P = 0.1381). Several factors may explain this result including a lack of trapping efficacy, design flaws, low intensity of trapping, and differences in island habitat affecting avian nest site selection and sample size. Nest success for both dabbling ducks (P = 0.0225) and Willets (P < 0.0001) was inversely related to predator activity, as measured by artificial nest success. In contrast, Canada Goose (P = 0.6686) showed no relationship between nest success and predator activity. For Canada Goose (P = 0.0064) and Willet (P = 0.0029), nest density decreased with increasing predator activity on island plots. Biased nest detection, philopatry to islands with reduced predation risk, and active selection for reduced predator environments may explain the higher nest density on islands with

Nest predation is a major determinant of fitness in birds and costly nest defence behaviours have evolved in order to reduce nest predation. Some avian studies have suggested that predator recognition is innate whereas others have stressed the importance of learning. However, none of these studies controlled for the genetic origin of the populations investigated and the effect of unfamiliarity with the predator. Here we determined whether experience with a nest predator is a prerequisite for nest defence by comparing predator recognition responses between two isolated but genetically similar Seychelles warbler (Acrocephalus sechellensis) populations, only one of which had experience of the egg predating Seychelles fody (Foudia sechellarum). Individuals in the predator-free population significantly reduced nest guarding compared to individuals in the population with the predator, which indicates that this behaviour was adjusted to the presence of nest predators. However, recognition responses (measured as both alarm call and attack rates) towards a mounted model of the fody were equally strong in both populations and significantly higher than the responses towards either a mounted familiar non-predator and a mounted, novel, non-predator bird species. Responses did not differ with a warbler's age and experience with the egg predator, indicating that predator recognition is innate.

We used presentations of models to determine the effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens against a nest predator (Blue Jay Cyanocitta cristata) and a brood parasite (Brown‐headed Cowbird Molothrus ater). Principal components analysis (PCA) of four component variables of nest defence (call rate, swoop rate, closest approach and number of adults) generated a measure of overall nest defence (aggression). We determined effectiveness of defence by looking for correlations between measures of defence and measures of nest success (nest predation and brood parasitism). We also determined whether nest defence increased with clutch size, nestling age and time in the breeding season. Defence against model Brown‐headed Cowbirds did not correlate with levels of parasitism, clutch size, age of young or time of breeding. There was, however, a strong, but insignificant, trend for nests with high levels of all measures of defence to suffer less from brood parasitism. Aggression, vocalization rate, closest approach and number of adults defending against models of predatory Blue Jays correlated positively with nesting success during the egg stage but not the nestling stage of the nesting cycle. Aggression, vocalization rate, closest approach correlated with clutch size and age of the brood. These results suggest that nest defence can effectively deter nest predators, but may be less effective against brood parasites. Different behavioural components of nest defence may work at different stages of the nest cycle and against different nest predators. The components of nest defence that correlated with nest success also correlated with clutch value, a result consistent with hypotheses on the evolution of nest defence.