A 2000 study hypothesized this to be because males have more variable reproductive success, and therefore that a high quality male produces more offspring than a female of similar quality. The growth of nestling tree swallows is influenced by their environment. In both younger and older nestlings (those between two and four days old and between nine and eleven days, respectively) growth is positively influenced by a higher maximum temperature, particularly in the former. A later hatching date negatively impacts growth, especially for younger nestlings. Older chicks grow somewhat faster when insects are abundant. Growth in younger nestlings increases with age, while in old nestlings, it decreases as they get older. Young tree swallows are able to thermoregulate at least 75% as effectively as the adult at an average age of 9.5 days when out of the nest, and from four to eight days old when in the nest (depending on the size of the brood). The nestlings fledge after about 18 to 22 days, with about 80% fledging success. Like hatching success, this is negatively affected by unfavourable weather and a younger female. Chicks may be preyed on by snakes and raccoons. This predation can be exacerbated by begging calls. The tree swallow forages up to above the ground singly or in groups. Its flight is a mix of flapping and gliding. During the breeding season, this is mostly within of the nest site. When it is foraging for nestlings, though, it usually goes up to from the nest, mostly staying in sight of it, and forages at a height up to . As well as being caught in flight, insects are sometimes taken from the ground, water, vegetation, and vertical surfaces. The tree swallow eats mostly insects, with some molluscs, spiders, and fruit. In North America, flies make up about 40% of the diet, supplemented with beetles and ants. Otherwise, the diet is about 90% flies. The insects taken are a mix of aquatic and terrestrial organisms; the former is an important source of omega-3 highly unsaturated fatty acids. This is because, although the tree swallow can convert the precursor α-Linolenic acid into high unsaturated fatty acids like docosahexaenoic acid, it cannot do so in the quantities needed. The seed and berry food is mainly from the genus "Myrica", which is mainly taken in all four of the Northern Hemisphere seasons except summer. Crustaceans were also found to be important in the wintering diet in a study on Long Island, New York.