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

Abstract

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 intensity of nest defense among colonial Black-tailed Gulls Larus crassirostris in two years. Only 30-40% of males attacked a decoy of an egg predator (crow), 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 intensity of nest defense.

Endocrine and neuroendocrine regulation of fathering behavior in birds

“…In European starlings, male testosterone levels correlated negatively with incubation behavior (Pinxten et al, 2007), but male black-tailed gulls (Larus crassirostris) with higher circulating testosterone during incubation were more likely to defend their nests against model predators than males with lower levels (Kazama et al, 2011). Collectively, these data suggest that the assumption that testosterone mediates a trade-off between sexual/ aggressive and paternal behavior is not universally applicable among birds.…”

Defending the Weak: Parental Defense Peaks When Chick Vulnerability is Greatest in the Herring Gull (Larus argentatus)

“…This lack of collective colony defense remains unexplained but suggests that collective action may be context specific. Sex-based differences in parental aggression may also exist (Kazama et al 2011). On average, we found a greater intensity of nest defense in response to the simulated predation events when both parents were present at the nest than when a single parent was present.…”

“…Group defense against predators is often considered an adaptive benefit of colonial nesting (Lack 1968). We expected, therefore, that neighboring birds may join in nest defense against a simulated threat of predation (Kazama et al 2011). Curiously, no neighboring individuals joined focal birds in antipredator behaviors during our trials.…”

“…Predators may be more effectively deterred when multiple individuals participate in defense (Caro 2005), and the risks of nest defense may be reduced in biparental mating systems relative to those of uniparental systems (Montgomerie & Weatherhead 1988). Sex-based differences in parental aggression may also exist (Kazama et al 2011). We were often unable to identify the sex of the defending parent(s) and are therefore unable to test for sex-based differences, but variability in nest defense behaviors between sexes provides an interesting avenue for further research.…”

Consistent individual variations in aggressiveness and a behavioral syndrome across breeding contexts in different environments in the Black-tailed Gull

“…The anti-predator aggressiveness of individual males is highly consistent within the incubation period (approximately 30 days) and across years Watanuki 2010, Kazama et al 2011). These consistent individual variations in anti-predator aggressiveness are independent either of the body size, clutch size, timing of egg-laying, or age of the clutch, indicating that the individual aggressiveness was not regulated plastically with those ecological factors Kazama et al 2011). In this species, males copulate with their mates and sometimes with other females, and defend more 5 aggressively against conspecific territorial intruders than females, while males and females share chick food provisioning equally .…”