Timing and nest aggression in Purple Martins (Progne subis)

Abstract

Earlier arrival at the breeding grounds after migration can confer advantages for migratory songbirds, including access to higher food abundance and to better compete for desirable territories. Selection for one sex to arrive before the other is often due to intrasexual competition for territories and mates, though the link between early female arrival (protogyny) and female-female competition is unclear from lack of research on female territoriality and nest aggression. I explored timing and aggression in the cavity nesting species Purple Martin (Progne subis), where males are known to defend nest cavities and compete aggressively toward other males though display protogyny, but these patterns have not been investigated in females. I investigated whether early arrival timing has selected for higher levels of aggression, competition for territory, and nest defense between females. I conducted a behavioural experiment at nesting colonies of Purple Martin in Winnipeg, Manitoba, where levels of aggression were assessed with simulated territorial intrusion using taxidermy mounts of female martins. I found that early nesting and age were positively associated with aggression, affirming that older birds that arrive and nest earlier are likely more experienced and in better condition to defend cavities. Sex had no significant effect on aggression, suggesting shared duty of defending nests by both members of the social pair regardless of the sex of the intruder. Females defended most aggressively after eggs were laid, which supports the parental investment hypothesis, where higher aggression is associated with higher nest investment. Overall, this study provides new insight on the role of age and timing on female aggression and nest defense in a migratory songbird, which will be valuable to further explore as migration timing advances with climate change.