Cooperative nest-defence behaviour and territory quality in a resident and socially monogamous passerine

Females Are More Aggressive Than Males towards Same- and Opposite-Sex Intruders in the Blue Tit (Cyanistes caeruleus)

During the breeding season, aggression is expressed to gain access to resources such as territories and mates and protect offspring. Female aggressiveness has received much less attention than male aggressiveness, and few studies have examined female and male aggressiveness towards intruders of both sexes in the same species. We compared female and male aggressiveness towards same- and opposite-sex intruders during the egg-laying period in blue tits (Cyanistes caeruleus) using simulated territorial intrusions. For each sex, we examined the occurrence of different behavioural responses during agonistic encounters, and compared the intensity and individual consistency of intra- and inter-sexual aggression using same- and opposite-sex taxidermy mounts. Our results show that females are the more aggressive sex. Both sexes showed similar behaviours during simulated intrusions, although females were never observed singing and males never entered the nest box. In females, aggression was predominantly independent of the sex of the intruder, while males sang more from a distance during male-male encounters. The relative levels of aggression (pecking and perching on the mounts) during intra- and intersexual conflicts were consistent for females, but not for males. Females might be under stronger selection for aggressive phenotypes due to nest-hole competition and larger reproductive investments.

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, 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.

Territorial Responses of Nuthatches Sitta europaea—Evaluation of a Robot Model in a Simulated Territorial Intrusion

In birds, aggressive behavior can be elicited in the field with a simulated territory intrusion (STI). Here, we compared four different STI methods in nuthatches in the non-breeding season: playback only, playback combined with an inactive wooden model mounted on a robot device, playback and an active model mounted on a robot device, and playback with the robot device only. In the control treatment, birds were not exposed to STI. Experiments were carried out in 12 territories. Behavioral observations included latency to approach, latency to start calling, time spent in the same tree, number of flyovers, minimum distance, number of individuals, number of flights into the tree, and number of calls. There was no significant influence of stimulus presentation. Nuthatches responded equally to all four experimental manipulations, but order of presentation had a strong influence.