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.