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.
Parents of barn swallows, however, may be looking for the healthiest throats so they can pop food into the youngsters most likely to survive, according to Nicola Saino of the University of Milan in Italy and her colleagues. In the Jan. 7 Proceedings of the Royal Society of London B, they propose that the gape redness of a begging nestling indicates whether the young bird is battling infection.
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Young barn swallow gapes vary from a greenish yellow to bright red, depending on the abundance of carotenoid pigments. In mammals, carotenoids play important roles in the immune system, and Saino hypothesizes that bird immune systems also involve carotenoids.
Thus, a nestling that’s battling infection might not have much pigment to spare for throat color, whereas a healthy nestling can flash a dramatic red.
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. These youngsters’ gapes paled beside those of untreated nestmates. When researchers dyed baby bird gapes with red or yellow food coloring, parents indeed gave the redder nestlings more of the food.