Dominant Birds Stay Leaner Than Their Subordinates, Study Finds

A new study has found that socially dominant birds are generally leaner than their subordinate peers of the same species, probably because they can eat when they want and don't face as great a risk of starvation.

ominant birds stay lean during the day and then pack on the fat just when they need it most - before a chilly winter night. Staying lean helps birds stay more maneuverable during attacks by predators. Lean birds also have more time during the day to watch for predators, rather than spending the bulk of their time looking for food.

Natural selection would like to keep a bird as thin as possible, but also have enough fat to get through the night," said Thomas Grubb, a co-author of the study and professor of evolution, ecology and organismal biology at Ohio State University.

By eating late, dominant birds can reduce their risk of predation without increasing their risk of starvation."

The research appears in a recent issue of the journal The Condor.

Subordinate birds have a less predictable food supply during the day because they must look for food in places that dominant birds wouldn't normally bother with, according to Grubb. Also, dominants can displace subordinates from food.

"It seems that subordinate birds must carry more fat during the day as an 'insurance policy,' making them more vulnerable to capture by hawks and other predators," he said.

Grubb and his colleagues used data taken from hundreds of birds of known age and sex during a 10-year period. They found that, overall, dominant birds carry less weight than subordinate birds in the same species.

The researchers used feeding, foraging and body weight data taken from birds caught alive in central and northeastern Ohio from November through March 1988 to 1997. They focused on three woodland species: the Carolina Chickadee, Tufted Titmouse and White-breasted Nuthatch.

In these birds, dominant status depends on gender and age. In each species, adult males top the hierarchy, followed by juvenile males, then adult females and, finally, juvenile females.

The researchers associated a bird's body mass to the time of day it was caught. Dominant birds remained lean throughout most of the day, while the subordinate birds tended to be heavier.

A bird can gain as much as 10 percent of its total body mass each day in fat. Gaining fat before nightfall can help birds survive in winter because they often go into hypothermia as a survival mechanism. Birds can lower their body temperatures by as much as 42 degrees. (The average body temperature for these birds was about 104 degrees.) During the night, birds tend to lose their accumulated body fat.

"Extra fat at roosting time means a bird needs to go less far into hypothermia at night because it has more energy for metabolism," Grubb said. "Hypothermia is thought to be a cost, because it makes a bird less aware of its surroundings, therefore increasing its vulnerability to nocturnal predators."

Grubb co-wrote the study with Vladimir Pravosudov, a postdoctoral researcher at the University of California, Davis; Paul Doherty Jr., a graduate fellow in evolution, ecology and organismal biology (EEOB) at Ohio State; C.L. Bronson, a graduate research associate in EEOB; Elena Pravosudov, of the American River College in Sacramento; and Andrew Dolby, a visiting assistant professor at the University of South Florida in Tampa.