Many avian studies have documented how birds respond – both behaviorally and physiologically – to stress. Researchers most commonly describe predator–prey interactions from the adult birds' perspective. Birds exhibit a range of aggressive and defensive behaviors in response to threat. In many species, such overt displays have been linked directly to hormonal changes. For example, in an attempt to maintain a breeding area, the male bird will continuously patrol his claimed territory, present ‘threat displays’ such as very rapid wing beats, and sing repetitively when intrusion threats occur. Furthermore, each set of these behavioral responses has been shown to be influenced by testosterone levels.
A recent study by Ibáñez-Álamo, Chastel and Soler published in General and Comparative Endocrinology offers a new perspective on direct hormonal responses to stress through their analysis of corticosterone and testosterone levels in young ‘nestling’ birds.
As has been observed in adults, chicks too modify their behavior when threatened. By suppressing begging behavior, reducing vocalizations and calls for food/parents, young birds reduce their exposure to immediate threat from outside the nest. Ibáñez-Álamo and colleagues asked whether hormonal levels in common blackbird (Turdus merula) nestlings also change to reflect this protective behavioral modification.
The team took into account the fact that certain hormones are affected by stress (including handling by researchers) and suggested two possible but opposing scenarios for hormonal changes within nestlings. Either corticosterone levels increase when chicks perceive a potential nest predator (previous studies have suggested that when exposed to an acute stressor, nestlings have reduced vocalization and locomotion and increased levels of circulating corticosterone) or, alternatively, corticosterone levels decrease (high levels of this hormone have been shown to be linked to begging – a risky behavior for chicks if a predator looms just outside the nest).
As the birds are well known to limit their begging behavior when a predator is nearby, the team played recordings of magpies (Pica pica), a known predator, in the nest's surroundings and collected blood samples from the youngsters to check their hormone levels. They also took blood samples from chicks that heard no recording, as they had found in an earlier study that playing no recording and playing recordings of non-predatory birds had similar effects on the nestlings' hormone levels. Finally, they compared the hormone levels of safe birds with the hormone levels of nestlings at risk.
Ibáñez-Álamo and coworkers found that blackbird nestlings in the high-risk situation had lower corticosterone levels than the birds that were not at risk, fitting the prediction that lower corticosterone reduced begging behavior and thus reduced detection by a predator. They also detected higher testosterone levels in the chicks exposed to magpie calls compared with the low-risk condition, fitting the prediction that increased testosterone contributes to a reduction in begging behavior in this species.
While it is still unclear whether the hormonal changes measured here in chicks were triggered directly by predator cues or by the nestlings' parents' alarm calls, these findings offer insight from a novel perspective – nestling physiology – into the physiology of survival.