When fighting an infection, most animals mount a whole suite of physiological responses, ranging from specialist immune cells that combat the invader, to a variety of behavioural responses, known as sickness behaviour. However, activating an immune response can be an expensive exercise, so at certain times of year animals cut their costs by reducing their immune function. But do they modulate their sickness behaviour seasonally too? Noah Owen-Ashley and John Wingfield decided to test Northwestern song sparrows'sickness behaviour responses at different times of the year to see whether they modulated the behaviour seasonally(p. 3062).

Injecting male songbirds with bacterial lipopolysaccharide to simulate a bacterial infection in the winter and spring, the team monitored their aggression levels 24 hours later to see how the animals faired. During the winter, the treated males lost interest in defending their territory and even lost weight; their sickness behaviour was strong. However, during the spring when breeding occurs the treated males were every bit as feisty as the untreated males; they seemed to have lost their sickness behaviour. The spring males were modulating their sickness behaviour seasonally, probably because they had higher stress hormone levels that suppress the immune system and lower energy reserves than the over-wintering animals. Indulging in a spot of sickness behaviour is probably a luxury that breeding males can't afford.

Owen-Ashley, N. T. and Wingfield, J. C. (
). Seasonal modulation of sickness behavior in free-living northwestern song sparrows (Melospiza melodia morphna).
J. Exp. Biol.