The cerulean warbler undertakes an impressively long migration considering its small size, covering approximately 2,500 miles as it flies south between July and September (3) (5). The journey has to be completed once again at the end of winter, with the warbler reaching the breeding grounds in April and May (3).
Arriving at the breeding grounds a week earlier than the females, the males use this time to establish and maintain a territory. While this is achieved primarily by singing, aggressive physical attacks can occur (3), with males colliding in mid-air and grappling with bills and feet whilst they spiral towards the ground (2).
Cerulean warblers are apparently monogamous and so once the females arrive on the breeding grounds, pairs are formed, and the female sets about building a nest high in the canopy. This is generally a neat cup structure formed from shreds of bark, lichen, moss, and grass, bound together with spider webs and lined with hair and fine moss stems. Finally, the outside of the nest is often decorated with fragments of any grey or white material (2) (3). Usually each pair produces only one brood each year, laying an average of four eggs which are then incubated by the female for 11 to 13 days (3). The young are born with their eyes closed and are incapable of leaving the nest, and so are fed by the parents for the first 10 to 11 days of life (3). Cerulean warblers can first breed at the age of one, and may live for up to six years (2).
Cerulean warblers are insectivorous birds, feeding on a variety of insects as they hop along twigs examining the surfaces of leaves and bark for any potential prey (3) (2). They also sometimes consume small amounts of plant material (2).