Seasonal differences in baseline innate immune function are better explained by environment than annual cycle stage in a year-round breeding tropical songbird

1. Seasonal variation in innate immunity is often attributed to either temporal environmental variation or to life history trade-offs that arise from specific annual cycle stages but decoupling them is difficult in natural populations. 2. Here, we effectively decouple seasonal environmental variation from annual cycle stage effects by exploiting cross-seasonal breeding and moult in the tropical Common Bulbul Pycnonotus barbatus. We test how annual cycle stage interacts with a key seasonal environmental variable, rainfall, to determine immunity at population and individual level. If immune challenge varies with precipitation, we might expect immune function to be higher in the wet season due to increased environmental productivity. If breeding or moult imposes resource constraints on birds, depending on or independent of precipitation, we might expect lower immune indices during breeding or moult. 3. We sampled blood from 818 birds in four annual cycle stage categories: breeding, moult, ...