Avian life in a seasonally arid tropical environment: adaptations and mechanisms in breeding, moult and immune function

There is a growing appreciation of the diversity and uniqueness of tropical organisms, but evidence about the selection pressures that shape this diversity remains sketchy. In this thesis, I investigated how variation in life history traits arises from tropical environmental seasonality, starting with exploration of the annual cycle of the Common Bulbul Pycnonotus barbatus in Nigeria. This revealed that breeding was not seasonal, but moult was timed almost entirely to the wet season with only few individuals moulting into the dry season, suggesting that breeding and to a lesser extent moult, are decoupled from season, allowing me to test how breeding and moult affect immune function independent of seasonal conditions. On testing this, I found that season explained variation in immune function better than breeding or moult, but unexpectedly, immune indices were higher in the dry season and during breeding, contrary to expectations that infection risk is higher in the wet season, and that breeding constrains resources for immune function. Furthermore, I compared immune indices along the cool-wet to hot-dry environmental gradient in Nigeria and found that immune indices were rather similar. I then tested the effect of diet alteration on immune function, body mass and moult of bulbuls in captivity and found that bulbuls fed on fruits had better immune function, body mass and moult than those fed on invertebrates. These findings together suggest environmental conditions affect immune function more directly than via resource allocation trade-offs, but that variation in immune function does not follow simple environmental productivity pattern