When it鈥檚 time to procreate, northwestern song sparrows don鈥檛 let a little sickness get them down.
When breeding is in full swing in summer, the birds will ignore bacterial infections and continue to fend off territorial invaders as well as keeping up a constant weight. In winter, however, sick sparrows eat less and burn fat reserves to fight off the disease, while paying less attention to defending their territory. It鈥檚 the first time that animals have shown seasonal 鈥渟ickness behaviour鈥� in the wild.
Noah Owen-Ashley and John Wingfield of the University of Washington, Seattle, injected the song sparrows (Melospiza melodia morphna) in summer and winter with a lipopolysaccharide (LPS) that mimics a minor bacterial infection. Control birds received a saline injection. Birds treated with LPS became less aggressive in winter but not in summer (Journal of Experimental Biology, vol 209, p 3062).
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The researchers suspect that a number of hormonal factors, including high testosterone, may help suppress sickness behaviour, allowing the birds to keep up their weight and aggression.
