Breeding biology of the grey-browed brushfinch Arremon assimilis (Aves: Passerellidae) in the Eastern Andes of Colombia

The timing of breeding is a key component of the life history of birds. In the tropics, where avian diversity is higher and environmental variability is less predictable than in temperate latitudes, breeding phenology patterns are poorly understood, and many other life-history traits remain largely unstudied. We describe the nest, egg, juvenile plumage, and breeding phenology of a population of the grey-browed brushfinch (Arremon assimilis assimilis, Passerellidae) from the Eastern Andes of Colombia. We found a single bluish-white unmarked egg in a cup-shaped nest. We also examined the relationship between photoperiod and precipitation with breeding phenology by monitoring 44 territorial pairs between 2017 and 2018. Arremon assimilis has two breeding peaks associated with the onset of rainfall in prior months: the first peak extends from mid-May to September, and the second from November and December to January. We also update a recent review of breeding biology data in Arremon and discuss how life-history traits may have evolved in light of the relationships among sparrows in this genus. Our study, one of few analyses of the breeding biology of a population in a montane species from the Andes, highlights the importance of such studies to further comprehend the general mechanisms that shape the breeding biology of tropical birds.