No matter how much effort a bird puts into preening, there is always a time when its feathers need replacing. However, feather regrowth is a costly process. Jamie Cornelius from the University of California Davis, USA, is intrigued by the metabolic trade-offs that animals make between competing physiological processes. `Trade-offs are a very potent evolutionary force in forming the behaviour and physiology of animals,' she says. Knowing that animals divert resources from non-essential to critical processes, Cornelius and her colleagues wondered how resource-intense moulting impacts on the ability of some songbirds to deal with stress. She explains that most songbirds only moult after the breeding season and they free up resources for feather regrowth by suppressing their stress response. But how do nomadic songbirds that moult over a longer period and depend on unreliable food sources juggle the competing demands of their stressful lives versus feather regrowth? Knowing that corticosterone is the key hormone triggering a stress response in birds, Cornelius and her supervisor, Thomas Hahn, decided to measure levels of the hormone in red crossbills and zebra finches to find out how well they prepare for stressful encounters while replacing their plumage (p. 2768).
Unfortunately, capturing moulting red crossbills was far from easy. `The tricky thing is that red crossbills are nomadic so you don't know where they are going to be in a given year,' explains Cornelius. However, by tracking the bird's favourite food source – pine cones – as they ripened, Cornelius eventually located flocks of the elusive birds and was able to trap them by luring them into mist nets with captive red crossbill decoys. Then Cornelius had to work fast. Knowing that being trapped was stressful for the birds, she had to collect the first tiny blood sample from each bird within 3 min: before its corticosterone levels began rising. Next, she gently placed the bird in a cloth bag and collected three more blood samples over the course of an hour as the bird coped with the stressful situation. Finally, she assessed how far each individual's moult had progressed before releasing it back into the wild.
Meanwhile, down in Australia, Nicole Perfito was also trapping zebra finches, collecting blood samples and checking how may feathers they had replaced. `Zebra finches show a very slow moult that takes nearly a whole year,' explains Cornelius, who adds that the birds living in the deserts around Alice Springs are also dealing with very unpredictable conditions, so a strong stress response would be very beneficial.
Next, Cornelius travelled to Creagh Breuner's lab at the University of Montana to find out how the birds had handled the stress. Measuring the concentration of corticosterone in the trapped birds' blood, Cornelius found that the level of the stress hormone had rocketed and she also found that the zebra finches from the harsh desert around Alice Springs produced the highest corticosterone levels.
The nomadic birds with extended moults were able to maintain their costly stress responses for protection from their stressful lifestyle, unlike other songbirds that suppress the response while replacing their plumage rapidly. Having found that some songbirds do make the trade-off while others don't, Cornelius and her colleagues suggest that instead of being hardwired, songbirds may be able to adjust their moult/stress response trade-off to match their circumstances.