Birds in the wild frequently confront stressful situations that require a quick response to ensure survival. In these times of stress, a bird's body responds by releasing powerful hormones called glucocorticosteroids.
These hormones allow the bird to redirect energy from normal processes to respond to the emergency situation.
So for example, the bird is able to take immediate steps to evade or confront to a predator, but only by taking energy away from everyday processes such as feather growth.
Scientists have known for some time that a bird's response to stress can be measured by examining the level of a particular glucocorticosteroid called corticosterone. This hormone level varies between bird species and even closely-related subspecies of birds, but it isn't clear why. Environmental differences in altitude, predators, habitat, and climate can all affect the corticosterone stress response in very similar subspecies of birds.
But what would happen if the birds were raised in the exact same environmental conditions? Would the birds exhibit the same corticosterone levels in response to stress? Or are there other selection or developmental factors at work? The scientists at the Smithsonian Migratory Bird Center and several universities designed an experiment to answer these questions.
For this study, the team chose two closely related subspecies of swamp sparrows, the inland subspecies Melospiza georgiana georgiana found in Maryland and the tidal marsh subspecies Melospiza georgiana nigrescens found in coastal Delaware. The scientists collected four-day-old nestlings from the wild and raised them at the Smithsonian National Zoological Park in Washington, DC under the same environmental conditions for one year.
Tidal marsh subspecies on left, inland on right.
The scientists tested the birds' baseline and corticosterone stress response levels during their molt and after the first breeding cycle was complete. They took a small blood sample from each bird to establish baseline levels. These initial levels for all birds, regardless of subspecies or sex, were very low.
Scientists induced stress by keeping the birds in opaque cloth bag for 30 minutes, after which another blood sample was taken. These secondary test results showed there was little difference between the stress-induced corticosterone levels found in the female sparrows or the inland males, but the corticosterone levels were much higher in the male tidal marsh sparrows.
The experiment showed that closely related subspecies of birds exhibit different hormone responses to stress when raised in the same environmental conditions. This indicates that the corticosterone response is not strictly influenced by environmental conditions. Rather, the difference in the stress response of these two very closely related birds is influenced by evolutionary and selection processes as well.
Even though the team began rearing the chicks 4 days after hatching, there is also still a possibility that some event or condition in those first four days in the wild could have also affected the developmental processes related to the birds' stress responses.
Since male tidal marsh sparrows are known to be more aggressive than inland males, the team is also considering the relationship between an increased stress response and aggressive behavior. To deal with a stressful situation, a bird must often give up its claim on resources such as food and redirect its energy into dealing with the immediate situation at hand.
The team reasons that since a more aggressive bird would not have difficulty reasserting itself and reclaiming resources after the stressful situation is resolved, it would be able to temporarily expend its energy on a high-level stress response. For a shy bird, a high stress response may not pay off in the end. The more timid bird may not be able to reassert itself and regain the resources it gave up to mount the strong response to stress.
This article summarizes the information in this publication:
Angelier, Frederic, Ballentine, B., Holberton, R. L., Marra, P. P. and Greenberg, R. 2011. What drives variation in the corticosterone stress response between subspecies? A common garden experiment of Swamp sparrows (Melospiza georgiana). Journal of Evolutionary Biology, 24(4): 1-10.
Although differences in the corticosterone stress response have frequently been reported between populations or closely related subspecies, their origin remains unclear. These differences may appear because individuals adjust their corticosterone stress response to the environmental conditions they are experiencing. However, they may also result from selection that has favoured individuals with specific corticosterone stress response or from environmental factors that have affected the development of the corticosterone stress response during early life. We investigated these hypotheses by studying the corticosterone stress response of two closely related subspecies of swamp sparrows (Melospiza sp.). We showed for the first time that two closely related subspecies can differ in their corticosterone stress response when raised at the laboratory and held in similar conditions for a year. Thus, we demonstrated that selection, developmental processes or a conjunction of both of these processes can account for variation in the stress response between closely related subspecies.
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