Nest defense by Mountain Bluebirds (Sialia currucoides) is related to their sex and brood characteristics but not morphological attributes

Within species, bird parents defend their nests with different levels of intensity. Previous studies have examined several hypotheses for this variation by looking at factors such as morphological characteristics of the parents and the characteristics of the brood, but few have controlled for multiple factors simultaneously. Here we examined a broad range of factors that may influence the nest defense behaviour of Mountain Bluebirds (Sialia currucoides Bechstein, 1798), including the adult’s morphology (sex, age, plumage colour, body size, and body condition) and characteristics of their brood (size, hatch date, stage of nestling development). Parental aggression was triggered using a taxidermy mount of an American red squirrel (Tamiasciurus hudsonicus Erxleben, 1777). We found no relationship between nest defense and any morphological characteristic of adults but the intensity of nest defense was positively correlated with the stage of offspring development, clutch size, and hatch date consistent with parental investment theory. Males also defended nests more strongly than females, consistent with the premise of the renesting hypothesis that males have fewer future opportunities to breed and therefore are more invested in the current brood. Our results provide strong support for parental investment theory and renesting as the primary drivers of nest defense intensity in breeding birds.