Shading by vegetation facilitates cryptic reproductive behaviour in a tropical songbird

Vegetation structural complexity surrounding nests can either provide concealment for intruders and mates or make it more difficult for hosts to recognize parasitic eggs. We investigated whether shading and vegetation aggregation increase extrapair paternity (the presence of broods with half-siblings) and intraspecific brood parasitism (IBP, the occurrence of broods with unrelated offspring) in socially monogamous Blue-black Grassquits Volatinia jacarina. We found that habitat shadowing was associated with increased occurrence of IBP, but found no association between the degree of shade and the presence of extrapair offspring. Our findings support the idea that habitat limits cryptic reproductive behaviours and that female grassquits may benefit from habitat shadows to parasitize conspecific nests.