Horn of Dalhousie University compared the power of begging squeaks from tree swallow nestlings that had been deprived of a meal or had been recently fed.
The scientists created a short-term need by keeping otherwise healthy nestlings from getting a meal.
We studied diets of nestling tree swallows in an upland old field in western Michigan to: (1) determine the types and proportions of prey items fed to nestlings by adults, (2) investigate the relationship between the number of prey items per bolus and bolus weight and brood size and (3) investigate the relationship between bolus composition and weather.
In 1997 we assessed the diets of nestling tree swallows reared in wooden nest boxes mounted on metal poles erected in a grid in an upland old field on the campus of Grand Valley State University, Ottawa County, Michigan (42[degrees]57'N, 85[degrees]53'W).
When barn swallow nestlings open wide and say "Ah," parents may check their health much as human moms and dads peer anxiously down Junior's throat.
In Italian and Danish barn swallow colonies, the researchers challenged some of the nestlings' immune systems with a standard test--injections of sheep red blood cells.
The helpers' hormones were already on the rise before they encountered the nest or nestlings, "suggesting that high prolactin levels were not a simple response to the stimulus of begging young," Schoech and his colleagues report.
Parents had the highest prolactin counts and visited and fed the nestlings most often.
Gowaty theorized that the female nestlings
might need more food because they use up more energy than their brothers during the feeding process.