Comparative immunoecology in birds: hypotheses and tests

In this review, I focus on three key questions in avian comparative immunoecology: variation in immune responses in relation to sex; latitude (and pace-of-life); and the annual cycle. I present hypotheses and evaluate the so far rather scanty and heterogenic data to test them. Sex differences in immune responses have been hypothesized to be caused by inferior immune responses in the heterogametic sex (females in birds), sexual selection (males invest more in mate acquisition and less in immune function compared to females under polygyny, whereas the sexes invest equally in immune function under monogamy), or body size differences. Available data refute the heterogametic sex hypothesis, but tentatively support the sexual selection hypothesis. Latitudinal patterns of immune responses have been hypothesized to be adjusted to parasite pressure, pace-of-life or breeding season stress. In passerine birds, species breeding closer to the equator (where parasites presumably are more abundant) tended to show stronger humoral but not cell-mediated immune responses. Annual patterns of immune responses could be related to melatonin levels or adjusted to seasonal differences in parasite exposure (high exposure in tropical migrants in winter and in temperate breeding birds in summer). The results from studies of immune responses over the annual cycle in birds show no clear pattern over the annual cycle and there is little consistency between different components of the immune system. Clearly, to facilitate further testing of these intriguing ideas in comparative immunoecology, more studies on non-domesticated birds are needed.