Radiotelemetry reveals variation in fever and sickness behaviours with latitude in a free‐living passerine

Summary 1.  The acute phase immune response, which includes fever and sickness behaviours, carries high costs in energy and time, but enhances pathogen clearance in diverse hosts. Hypotheses based upon pathogen pressures and life‐history trade‐offs predict that costly immune responses will decrease in strength as latitude increases. However, whether the acute phase response shows latitudinal patterns among free‐living, wild populations remains unknown. 2.  Here, we studied fever and sickness behaviours during the early breeding season in free‐living song sparrows ( Melospiza melodia ) along a latitudinal gradient in southern California (CA), Washington (WA), and Alaska (AK). In 2007 and 2008, we injected males with lipopolysaccharide and assessed sickness behaviour by measuring changes in territorial aggression. In 2008, we monitored fever and sickness behaviour in CA and WA birds using a novel telemetric technique: skin‐mounted radiotransmitters with temperature sensors. 3.  In 2007, territorial defence varied by latitude, with a lower probability of territorial response at 24 h after injection in CA, but not in WA or AK. Radiotelemetry in 2008 revealed that CA birds showed pronounced and prolonged lethargy and fever ( c . 2 °C above control males throughout the night), whereas WA birds showed only moderate lethargy and fever ( c . 1 °C, returning to control levels during the night). 4.  This study establishes radiotelemetry as a powerful method for quantifying fever and sickness behaviours in small, free‐living vertebrates. Moreover, our data suggest that latitude predicts the strength of these responses. These results can provide insight into disease susceptibility and spread among wild populations.