Intensity Of Nest Defence Research Articles

Nest defense by Mountain Bluebirds (Sialia currucoides) is related to their sex and brood characteristics but not morphological attributes

Within species, bird parents defend their nests with different levels of intensity. Previous studies have examined several hypotheses for this variation by looking at factors such as morphological characteristics of the parents and the characteristics of the brood, but few have controlled for multiple factors simultaneously. Here we examined a broad range of factors that may influence the nest defense behaviour of Mountain Bluebirds ( Sialia currucoides (Bechstein, 1798)), including the adult's morphology (sex, age, plumage colour, body size, and body condition) and characteristics of their brood (size, hatch date, and stage of nestling development). Parental aggression was triggered using a taxidermy mount of an American red squirrel ( Tamiasciurus hudsonicus (Erxleben, 1777)). We found no relationship between nest defense and any morphological characteristic of adults, but the intensity of nest defense was positively correlated with the stage of offspring development, clutch size, and hatch date consistent with parental investment theory. Males also defended nests more strongly than females, consistent with the premise of the renesting hypothesis that males have fewer future opportunities to breed and, therefore, are more invested in the current brood. Our results provide strong support for parental investment theory and renesting as the primary drivers of nest defense intensity in breeding birds.

Variation in alarm calls during different breeding stages of the common kestrel (Falco tinnunculus).

ABSTRACTAcoustic signals play a key role in animal communication. Animals usually use alarm signals to warn mates or offspring of the presence of threats or to intimidate or distract predators. Birds commonly use acoustic signals as a means of communication. Alarm calls in passerines at different breeding stages can reflect their nest defense intensity. However, little is known about the characteristics, plasticity, and impact factors of alarm calls during the reproductive period in raptors. Here, from March to July in 2019, the alarm calls of eight pairs of common kestrels (Falco tinnunculus) during the breeding period were recorded using a portable recorder with a strongly directed microphone in the Zuojia Nature Reserve, Jilin province, China. The differences in acoustic parameters of parental alarm calls in different breeding stages were analyzed. The results showed that the alarm calls of common kestrels were composed of multi-harmonic arched frequency modulation with the maximum energy distribution in the second harmonic. The duration and rate of the alarm calls increased significantly as the breeding season progressed, showing that parents spent increasing amounts of time on nest defense. Additionally, the acoustic parameters of alarm calls in common kestrels were significantly different depending on offspring numbers, suggesting that offspring numbers influenced parental nest defense. These results showed that differences in alarm calls during different breeding stages may reflect a trade-off between defense costs and reproductive benefits.

The effect of offspring value on variation of nest defense intensity in gray catbirds (Dumetella carolinensis)

Nest defense is an adaptive strategy used by adults to increase survival of independent young. However, the risk adults face by defending young from predators represents an important trade-off and parental investment theory predicts adults will modulate nest defense based on the value of offspring. We used a model of a black rat snake, a potential nest predator on our study site, to quantify nest defense in a population of gray catbirds in eastern Pennsylvania during the breeding seasons of 2017 and 2018. Specifically, we evaluated (1) the difference in nest defense between male and female parents, (2) the relationship between nest defense and nest success, and (3) how variation in nest defense was related to factors associated with offspring value. Although male catbirds tend to be larger than females, they did not defend their nests more aggressively than female catbirds. Response to the model predator we used was also not different between individuals whose nests were successful compared with individuals that failed. Although nests that contained more and older young were not defended more aggressively, our analyses did show that adult catbirds defended young that were laid earlier in the season more intensely than those laid later in the breeding season. In birds, earlier clutch initiation is associated with an increased chance of recruitment into the breeding population. Catbirds, therefore, follow predictions of parental investment theory because they defended offspring of higher value more aggressively.Significance statementDefense of young from predators is an important component of parental care. Nest defense represents a trade-off because although it should increase reproductive success by increasing offspring survival, it presents a risk to adults, thus lowering their survival. We studied variation in nest defense of gray catbirds in relation to offspring value. Although nest defense was not related to nest success, we did find that adult catbirds defended nests that were produced earlier in the year more aggressively than later nests. In birds, the timing of reproduction is linked to prospects of juvenile survival in such a way that offspring produced early in the breeding season are more likely to survive and recruit into the breeding population. As predicted by parental investment theory, catbirds defended young that represented a higher increase in fitness more aggressively.

Similarity in nest defense intensity in Canada goose pairs

Individuals consistently differ in their behavior, and such personality differences are often linked to fitness. For biparental care species, the behavior of both parents can impact reproductive success. Behavioral similarity within pairs has been shown to provide selective advantages for many species. In this study, we tested for behavioral similarity in 129 pairs of nesting Canada geese (Branta canadensis) with respect to nest defense behavior, an individually consistent trait. We used a bivariate mixed model to provide unbiased estimates of correlations between partners’ traits. We also tested for the effect of behavioral similarity between mates on reproductive success and predicted greater success for positively assorted pairs. We found that over four consecutive years, pairs of Canada geese were formed of individuals showing similar nest defense intensities. Assortment between mates may result both from personality-dependent non-random mate choice and from behavioral adjustments after pairing that are maintained in the long term. However, behavioral adjustment to a shared environment, among and within breeding seasons, had negligible effects on the covariation between partners’ traits. We also found that similarity in nest defense intensity did not result in higher reproductive success. The factors at the origin of the evolution of long-term behavioral similarity based on nest defense in Canada geese pairs remain to be identified. Individual consistency in behavioral traits has been demonstrated for many animal species, and its importance in ecology and evolution is now accepted. The maintenance of personality traits in a population may in fact be explained by non-random mate choice like assortative mating, where paired individuals show similar phenotypes. We studied the nest defense behavior, a highly repeatable trait, of 129 Canada goose pairs during four consecutive years and found that paired individuals had similar personalities, suggesting personality-dependent mate choice or post-pairing adjustments maintained in the long term. Our findings also show that the phenotypic similarity between mated individuals is not always related to reproductive success and can possibly result from non-adaptive mechanisms.

Can parental investment reduce social altruistic behaviour in Yellow-legged Gulls Larus michahellis

This study aims to test if there are differences in nest defence behaviour in single and in groups of Yellow-legged Gulls Larus michahellis during two stages of the breeding season: incubation and early chick-rearing period. When a human intruder wearing a mask approached and stood still next to the target nest during incubation, the gulls took part in passive mobbing and helped the ‘attacked’ gull, thereby showing altruistic behaviour. In contrast, during the early chick-rearing period, decreased altruistic behaviour was observed: the individuals that took part in the passive mobbing tended to remain on their own nests, in order to look after their own chicks. In this stage, a reduction of the size of passive mobbing was also noted. Furthermore, gulls from disturbed nests increased the intensity of nest defence by increasing the number of dives directed at the heads of the human intruders. Again, those gulls whose nests were directly affected by an approaching human intruder left their nests quickly to begin an aerial defence, encouraging the chicks to leave the nest and hide among rocks and shrubs. The adult gulls came back to their nests only after the danger had ceased and the chicks had come back to their nests, in agreement with the Parental Investment Theory.

Colony defensive behavior by the swarm-founding wasp Parachartergus pseudoapicalis: increase on investment predicts the intensity of nest defense

The parental investment (PI) theory predicts that animals that have the ability to care for their offspring should defend them with an optimal intensity to minimize the risks of predation. This study evaluates the stages of development that are more valued and, therefore, are more protected by the colonies of the founding wasp Parachartergus pseudoapicalis. We determined the effects on the colony defense intensity of colony size, adult population, and immature population size for different stages. As predicted by PI theory, we found that defense intensity was related to colony investment in offspring. In addition, we found no association between the number of adults in the colony and the defense intensity, whereas a greater number of immatures in the pupal stage in the colony led to greater defense intensity. Our study highlights the importance of parental care in social insects and the changes in defense behavior present in colonies with different compositions of individuals at different stages.

The nest defence by the red‐backed shrikeLanius collurio– support for the vulnerability hypothesis

The majority of altricial bird species defend their brood against predators more intensively in nestlings rather than eggs stage. Several hypotheses have been proposed to explain this difference. The majority of existing experimental studies have recorded a gradually increasing intensity of nest defence supporting the reproductive value hypothesis. We have compared nest defence in two nesting stages of the red‐backed shrike against two predators of adult birds and against two predators of nests. While the nests with nestlings were defended by parents against three out of four predators, nests with eggs were almost not defended at all. This rapid change in parent nest defence supports rather the vulnerability hypothesis, predicting that the threat to nests with nestlings increases rapidly after hatching, as they became more conspicuous due to their begging and parental provisioning. Unlike most of the species tested previously, the red‐backed shrike uses very vigorous mobbing towards predators. We suggest that the occurrence of this active mobbing (strikes, including physical contact) is a good proxy of the current threat to the nest.

Age-related parental care in a long-lived bird: implications for offspring development

In long-lived species, behaviour is expected to vary across the lifespan, first improving with maturation and experience and then declining with senescence, but measurement is rare, particularly during old age. Measuring nest defence intensity in 255 known-age blue-footed boobies (Sula nebouxii) of 19 age classes confirmed the inverted-U pattern: intensity of nest defence increased in young males and females until roughly 11–12 years and declined progressively afterwards. We discuss the processes that may generate this distinctive pattern. Regardless of the underlying processes, this study establishes the existence of age-related variation in parental care in a long-lived species. Natural variation in natal conditions has recently been shown to have long-term impacts on development, so this study raises the question whether age-related changes in parental care generate systematic variation in offspring phenotypes. In theory, age-related change in parental care can result in differences among offspring produced at different ages. However, parental behaviour has never been measured over the whole age-span of any long-lived species because of the difficulty of sampling very old animals in the field. We measured intensity of nest defence over the age-span and confirmed that blue-footed boobies exhibit an age-related quadratic pattern in this parental behaviour: defence intensity increases in early life and declines progressively afterwards. Our results imply that similar-shaped patterns may exist in other components of parental care, such as feeding and social interaction, potentially affecting the long-term development of offspring.

Environmental heterogeneity and population differences in blue tits personality traits.

Environmental heterogeneity can result in spatial variation in selection pressures that can produce local adaptations. The pace-of-life syndrome hypothesis predicts that habitat-specific selective pressures will favor the coevolution of personality, physiological, and life-history phenotypes. Few studies so far have compared these traits simultaneously across different ecological conditions. In this study, we compared 3 personality traits (handling aggression, exploration speed in a novel environment, and nest defense behavior) and 1 physiological trait (heart rate during manual restraint) across 3 Corsican blue tit (Cyanistes caeruleus) populations. These populations are located in contrasting habitats (evergreen vs. deciduous) and are situated in 2 different valleys 25 km apart. Birds from these populations are known to differ in life-history characteristics, with birds from the evergreen habitat displaying a slow pace-of-life, and birds from the deciduous habitat a comparatively faster pace-of-life. We expected personality to differ across populations, in line with the differences in pace-of-life documented for life-history traits. As expected, we found behavioral differences among populations. Despite considerable temporal variation, birds exhibited lower handling aggression in the evergreen populations. Exploration speed and male heart rate also differed across populations, although our results for exploration speed were more consistent with a phenotypic difference between the 2 valleys than between habitats. There were no clear differences in nest defense intensity among populations. Our study emphasizes the role of environmental heterogeneity in shaping population divergence in personality traits at a small spatial scale.

A classic question revisited in red-winged blackbirds: disentangling confounding hypotheses surrounding parental investment theory and nest defense intensity

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.

The multidimensionality of behavioural defences against brood parasites: evidence for a behavioural syndrome in magpies?

Studies of antiparasite defences against cuckoo parasites have largely neglected the possibility that behavioural components of host defence may correlate giving rise to a behavioural syndrome. Furthermore, the different contribution of the host’s sex in nest defence has traditionally been disregarded. Here, we studied magpie (Pica pica) mobbing behaviour towards dummies of great spotted cuckoo (Clamator glandarius) and non-harmful hoopoes (Upupa epops) and egg rejection of parasite eggs in a population of colour-banded magpies. We predicted a positive correlation between the intensity of nest defence and egg rejection within each sex and that females respond more intensely than males to the threat of brood parasitism as they undertake incubation. Magpie males, but not females, defended their nests more intensely in those nests in which cuckoo model eggs were rejected. Individual magpies did significantly differ in their baseline level of nest attentiveness; however, there were no individual differences once pair identity was considered. Males and females defended their nests more intensely when it was exposed to the presence of a great spotted cuckoo dummy. Males, but not females, were more prone to appear at their nests, and females, but not males, were more prone to defend more intensely when their nests were challenged by a parasite threat. Our results thus agree with the view that mobbing behaviour and egg rejection in magpies may actually constitute a pseudosyndrome and highlight the necessity to integrate interindividual variation and the sex of the host in studies of the evolution of host defences.

Fearing the feline: domestic cats reduce avian fecundity through trait‐mediated indirect effects that increase nest predation by other species

Summary Urban areas contain high densities of non‐native species, which in the UK include the domestic cat Felis catus (Linnaeus 1758) and the grey squirrel Sciurus carolinensis (Gmelin 1788). The direct predation effects of domestic cats on prey populations attract intense debate, and such influences of the nest‐predatory grey squirrel are receiving increasing attention. In contrast, theory predicts that sublethal and indirect effects are more important, but empirical evidence is currently lacking. We conducted controlled model presentation experiments at active urban blackbird Turdus merula (Linnaeus 1758) nests to provide the first empirical evidence that quantifies the potential sublethal and indirect effects of predators (domestic cat and grey squirrel) on avian reproductive success. Domestic cat models reduced subsequent parental provisioning rates, a strong indicator of sublethal effects, by one‐third relative to a nonpredatory rabbit Oryctolagus cuniculus (Linnaeus 1758) control. There was no compensatory increase in food load size. Previous experiments demonstrate that this magnitude of reduced food delivery will reduce nestling growth rates by c. 40%. The grey squirrel model induced similar but weaker effects. Following the brief presence of the domestic cat model, subsequent daily nest predation rates, chiefly by corvids, increased by an order of magnitude relative to the squirrel and rabbit models. The intensity of parental nest defence elicited in response to model presentations predicts the probability of such third‐party predator predation events, and the domestic cat model generated significant increases in nest defence behaviour. Synthesis and applications. The brief presence of a domestic cat at avian nest sites reduces subsequent provisioning rates and induces lethal trait‐mediated indirect effects. We provide the first robust evidence for these latter effects in any avian predator–prey system, although they are likely to be common in many avian assemblages with high predator densities. It is imperative that future assessments of the impact of predatory species on avian prey species take lethal trait‐mediated indirect effects into account, as so doing will prevent biased estimates of predator effects and facilitate the design of more effective control strategies. Full mitigation of the sublethal and indirect effects of domestic cats would problematically require permanent indoor housing.

Is shared male assistance with antiparasitic nest defence costly in the polygynous great reed warbler?

Polygyny typically has negative fitness consequences for secondary females, but may equally impose costs on primary females or even on polygynous males. We investigated how polygynous and monogamous great reed warbler, Acrocephalus arundinaceus , males assist their mates with aggressive nest defence against the common cuckoo, Cuculus canorus , and whether the females adjust their nest defence intensity according to male investment in aggression. Additionally, we investigated whether host social mating status affects host vulnerability to parasitism. We presented taxidermic cuckoo mounts at nests of primary, secondary and monogamous females, and recorded aggressive responses of nest owners. We found that monogamous males defended their nests most aggressively while polygynous males allocated their nest protection effort unevenly between their two mates, responding more vigorously on the primary than secondary nests. In contrast, nest defence intensity of females did not differ with respect to their social status, indicating that females of polygynous males did not compensate for low levels of male aggression. Similarly, we found no differences in natural cuckoo parasitism rates between monogamous, primary and secondary nests. Our results thus suggest that while monogamous females receive more assistance with nest defence than females of polygynous males, this has no effect on the probability of parasitism.

Comparison of nest defence behaviour between two associate passerines

Nest predation is one of the most important factors limiting reproductive success, and antipredator behaviour can significantly reduce the loss of avian broods. I carried out field experiments on two sympatric passerines: the barred warbler and the red-backed shrike. Many authors have described the protective nature of nesting association between these species. However, we have little knowledge about the true nature of the relationships between associates. I examined (1) whether barred warblers and red-backed shrikes respond differently to an avian predator, and (2) whether males and females differ in the intensity of nest defence. Decoys of a known nest predator and a non-predatory control species were used to examine the types and relative intensity of parental response. I measured behavioural responsiveness by recording aggressive behaviour toward each model during the nestling period. Barred warblers and red-backed shrikes showed considerable variation in their response. Warblers more vigorously defended their own territories than shrikes. No differences between the sexes in antipredator behaviour in red-backed shrike were found. By contrast, in barred warbler, male was more involved in nest defence. The experimental tests provide evidence that these two species are able to differentiate between a predator and non-predator species.

Parental Investment Theory and Nest Defence by Imperial Shags: Effects of Offspring Number, Offspring Age, Laying Date and Parent Sex

AbstractNest defence is a common form of parental care employed by birds to improve the survival of their offspring. Theory predicts that parents should adjust their nest defence according to the value of the brood at stake, defending more intensively broods with high survival and reproductive prospects. We evaluated the influence of offspring number, offspring age, laying date and parent sex on nest‐defence intensity (NDI) of the Imperial Shag Phalacrocorax atriceps, a sexually dimorphic seabird with seasonal decline in offspring survival and very limited renesting potential. We also evaluated whether NDI was correlated within pairs and whether NDI of both members of the pair was correlated with incubation and breeding success. To elicit defensive behaviour, we simulated predation attempts using a Kelp Gull Larus dominicanus model. As predicted by theory, NDI was positively correlated with the number of offspring in the nest and offspring age. NDI during chick rearing was higher than that at early and late incubation, while no differences were found between incubation stages. Contrary to our prediction, we did not find differences in NDI according to laying date. NDI for males was higher than females, while NDI was also positively correlated within pairs. NDI was not statistically related to incubation or breeding success. These results suggest that other factors, such as laying date or parental quality and age, play a much larger role in determining the outcome and productivity of a nesting attempt. Our results provide partial support for parental investment theory; while parental defence increased with brood value according to offspring number and age, parental defence was not related to laying date, a factor strongly affecting offspring survival and recruitment probabilities in this species.

Factors affecting individual variation in nest-defense intensity in colonially breeding Black-tailed Gulls (Larus crassirostris)

The physiological state of parent birds combined with the value of their clutch may affect the intensity of their nest defense. In colonially breeding birds, nest-defense intensity may also be affected by the behavior of neighbors. We investigated individual variation in the nest-defense intensity among colonial Black-tailed Gulls ( Larus crassirostris Vieillot, 1818) over 2 years. Only 30%–40% of males attacked a decoy of an egg predator (Large-billed Crow ( Corvus macrorhynchos Wagler, 1827)), and the other males and females rarely attacked. Males attacking the decoy had higher levels of plasma testosterone than males that did not attack. Each male’s, but not female’s, nest-defense intensity was consistent throughout the incubation period and also across years. The intensity was not related to egg-laying date, clutch size, or age of offspring. The intensity was likely to be higher when individuals had one or more neighbors, representing higher nest-defense intensity in the year where gulls had larger number of adjacent neighboring nests (5.23 nests), but this trend was not observed in the year where they had smaller number of the neighboring nests (3.73 nests). Thus, in addition to testosterone levels, behavior of neighbors also influences the nest-defense intensity.

Nest defence against avian brood parasites is promoted by egg-removal events in a cowbird–host system

Recent studies of birds have found that the antiparasite behaviour of host species is modified by social learning. We tested whether individual or social learning modifies the nest defence of yellow warblers, Setophaga petechia, against the parasitic brown-headed cowbird, Molothrus ater. Using field experiments, we exposed warblers to simulated events of nest parasitism and predation, or allowed them to observe conspecifics mobbing a cowbird. Intensity of nest defence by yellow warblers was greater after simulated threats at their nest than after they had observed mobbing of cowbirds by conspecifics. Warblers defended their nests more aggressively when they perceived a cowbird as an egg predator rather than as a parasite. Higher predation than parasitism frequencies probably explain the more prompt change in defence when cowbirds were encountered as egg predators. Annual fluctuations in frequencies of both parasitism and predation suggest that defence acquired via observing conspecifics may contain outdated information and probably explains the warblers’ reliance on individually learned over socially learned defence.

Seasonal increase in intensity of nest defence against little cuckoos by Japanese bush warblers

Avian brood parasitism reduces the reproductive success of hosts. Thus, hosts have evolved antiparasite strategies such as rejection of parasitic eggs and aggressive nest defence against brood parasites. Japanese bush warblers, Cettia diphone , do not reject the parasitic eggs of little cuckoos, Cuculus poliocephalus , because their eggs mimic warblers’ eggs. To examine whether Japanese bush warblers show nest defence behaviour against their parasite, I investigated the responses of the warblers to a little cuckoo dummy. Japanese bush warblers are residents, while little cuckoos are summer visitors; thus, the warblers start breeding earlier than the cuckoos. Accordingly, the risk of being parasitized increases as the season progresses. Therefore, I also examined whether the intensity of nest defence differed before and after cuckoo arrival. Japanese bush warblers showed remarkably aggressive responses to the cuckoo dummy, and levels of aggression were higher after the cuckoo arrived. These results suggest that Japanese bush warblers adaptively adjust their nest defence behaviour in response to increased risk of being parasitized. This study indicates that differences in migratory habits between hosts and brood parasites produce seasonal changes in antiparasite behaviour.

Flushing distances of ferruginous hawks nesting in rural and exurban New Mexico

As natural environments become increasingly modified by humans, land managers should devise plans to protect sensitive species from human activities that disturb these species. We explored behavioral responses of nesting ferruginous hawks (Buteo regalis) to an approaching human in areas where the level of human activity on the landscaped varied. Contrary to other reports, hawks were aggressive in nest defense and female hawks defended nests more aggressively than male hawks. Adult hawks decreased nest-defense intensity as nestling age increased but increased intensity with consecutive human visits to the nest. Flushing distance was considerably higher than that documented in other studies and was negatively related to degree of urbanization. We found that a distance of 650 m prevented 95% of nest-attending ferruginous hawks from flushing in response to human intruders; thus we recommend establishing a spatial buffer of this distance or greater to minimize adverse effects of human activities on hawks nesting in New Mexico. © 2011 The Wildlife Society.

Experimental Study of the Effect of Clutch Size on Nest Defense Intensity in Black-Tailed Gulls

Since parent birds are hypothesized to adjust the level of their nest defense against predators so as to enhance the survival of their offspring, parental nest defense is expected to increase in intensity in relation to increasing clutch size. Empirical studies of bird species, in which clutch sizes have been manipulated artificially, however, have produced results contradicting such expectations, partly, it is thought, because of various errors in experimental design, such as a lack of nesting habitat control, or providing too short a time for parents to assess the value of the manipulated clutch, Hence, further evidence needed to be gathered to clarify whether the basic hypothesis is adequately supported. We manipulated the clutch size of Blacktailed Gulls Larus crassirostris and observed the responses of male parents to a crow decoy in controlled nesting habitats. The intensity of defense was not affected by the clutch size. Opportunities for future reproduction, or constant individual levels of aggressiveness of seem to best explain the observed intensity of nest defense in these long-lived Black-tailed Gulls, rather than the value of the current clutch.