Latitudinal differences in the breeding phenology of Grey Warblers covary with the prevalence of parasitism by Shining Bronze-Cuckoos

Variation in the temporal patterns of nest availability through the breeding season or across the geographical range of a host is expected to be an important selection pressure shaping the breeding biology of avian brood parasites. The archipelago-wide distribution of the endemic Grey Warbler (Gerygone igata) in New Zealand, and its parasitism by the specialist Shining Bronze-Cuckoo (Chalcites lucidus), makes this a valuable system in which to study small-scale latitudinal gradients in host breeding phenology and the effects of these on the prevalence of brood parasitism. Nest records from throughout New Zealand and our study sites on both the North and South Islands indicated that, as expected, clutch-sizes were larger at higher, more southern, latitudes. Contrary to predictions, breeding began later and finished earlier, and usually involved only one brood on the North Island, compared with a longer breeding season with two broods on the South Island. Prevalence of brood parasitism covaried positively with latitude, suggesting that geographical patterns in breeding phenology of hosts may influence the prevalence of parasitism.