Eggs are laid from early May to mid-June (although this is happening earlier due to climate change) and chicks fledge between mid-June and July. Latitude is positively correlated with laying date, while female age and wing length (longer wings allow more efficient foraging) are negatively correlated. The tree swallow is likely an income breeder, as it breeds based on food abundance and temperatures during the laying season. This species is generally socially monogamous, but up to 8% of breeding males are polygynous. Polygyny is influenced by territory: males having territories with nest boxes at least apart are more likely to be polygynous. It is suggested that this polygyny depends on the conditions during the laying season: better conditions, such as an abundance of food, allow females in polygyny who do not receive help foraging to lay more eggs.