Rising to The Challenge: Mounting An Acute Phase Immune Response Has No Long-Term Negative Effects on Captive Sparrow Migratory Body Composition Or Migratory Restlessness.

Migratory animals may trade-off between investing energy in immune defence versus investing in energy reserves needed for seasonal migration. However, these trade-offs are often masked by other sources of variation and may not be detected through observational field studies of free-living animals. Moreover, observational studies can rarely distinguish the costs of pathogenic infection from those of mounting an immune response. To disentangle such effects, we conducted an immune challenge experiment. We captured song sparrows (Melospiza melodia) and white-throated sparrows (Zonotrichia albicollis) in autumn migratory condition, challenged the sparrows with non-infectious antigens that induce an acute-phase immune response, then monitored body composition and migratory restlessness behaviour. For both species, body mass was higher the day after exposure to keyhole-limpet hemocyanin (KLH) compared to controls. White-throated sparrows, but not song sparrows, increased lean mass one week after exposure to lipopolysaccharide (LPS), suggesting that effects of immune upregulation on body composition may be long-lasting and specific to certain combinations of hosts and antigens. White-throated sparrows exposed to KLH increased nocturnal migratory restlessness (Zugunruhe) for the week following exposure. These findings suggest that short-term activation of the acute immune response does not constrain migratory physiology in these songbirds.