January 31, 2014
When you go to the grocery store, you can choose whatever you like to eat. Wild birds, however, have a harder time browsing for their favorite foods. When birds DO succeed in finding enough food, they have to make a decision: invest in fattening up and replacing old feathers, or invest in raising a family. What should a bird do? This puzzle has fascinated many biologists, and it was exactly what Dr. Alexa Class Freeman, a biologist now at the Cornell Lab of Ornithology, set out to answer during her dissertation research in the Department of Biological Sciences at Virginia Tech.
The story, however, depends upon where the birds live. In temperate latitudes, times of plenty are short but predictable; every spring the predictable seasonal warmth leads to an abundance of food including seeds, insects and nectar. To a temperate breeding bird this signals pair bonding and nesting! In the tropics, seasons are less pronounced and food availability is more consistent. Tropical birds therefore don’t experience the same urgency to nest that occurs in seasonal or temperate climates. In response, tropical birds may show a suite of traits describing a more laid-back or ‘slow’ pace of life. These tropical birds can relax and have fewer and smaller clutches — but they also live a long time so they have plenty of opportunities to breed.
When biologists give temperate species supplemental (extra) food, those species almost always jump into breeding early or use the extra resources to have larger broods of young. But what happens when you give supplemental food to a tropical species? Dr. Class Freeman studied that very question in an Ecuadorian population of the rufous-collared sparrow, Zonotrichia capensis. Alexa provided food supplements to the sparrows and determined how they used the extra food resources: did they invest in self-maintenance (by molting) or reproduction (by nesting)? The answer: self-maintenance! Birds that were fed sent that extra energy right into their plumage, by initiating molt and replacing all of their worn feathers. Alexa’s research supported the suspicions of many biologists; that in the tropics, birds seem to have a slower pace of life.
For an analogy: if you suddenly win the lottery, do you invest big now (knowing that there may never come another chance), or do you invest in a slowly growing, safe stock fund that may benefit you in the long run? Birds in the tropics choose the less risky option. They minimize stress to self and increase longevity. This may seem selfish, but in the end it may allow them to live a long, fruitful life.