Breeding Biology


Female sitting on a nest to incubate her eggs (photo by N. Kovacs).

At the start of the breeding season, males arrive and establish territories, which are areas they defend by singing and by chasing out other males. Each warbler territory is a patch of forest that is used for foraging and nesting—birds get all the food they need from within their territories, and prevent other individuals of their species from feeding or nesting within that area of habitat. As in many migratory birds, females arrive a few days or a week after males have arrived and established territories, and each female pairs with a male for the breeding season. Like many birds, Black-throated Blue Warblers often return to the same territory year after year, where they may pair with their previous mate or with a new partner. The female isn’t as brightly colored as the male, and looks so different that early naturalists thought the two sexes were different species!

Females build the nests, lay eggs (usually 4), and incubate them without help from the males. They have a special section of their belly, called a brood patch, which has no feathers and lots of blood vessels. When they sit on their eggs, they place the brood patch on them so that their warm skin keeps the eggs nice and toasty.

Male feeding a young bird that just left its nest (photo by M. Cline).



It takes about 12 days for the eggs to hatch, and the nestlings emerge unable to do much more than swallow their food. At this point the male helps care for the young; both parents feed them continuously through the long days of summer, and in just nine days they grow to adult size. Once birds leave the nest they are called fledglings, and are able to hop about and fly short distances. The parents continue to feed them for about two more weeks, at which point they are able to find enough food on their own.

Typically, each warbler pair has multiple nesting attempts during the course of the breeding season. Some nests are lost to predators, and when that happens, the birds will quickly build another nest and start the process anew. Even when they are able to raise young successfully, they sometimes try a second time when enough food is available to them. In this case the male will continue to feed the young from their first nest, and the female will build a new nest, lay eggs, and incubate once again. This phenomenon is called double brooding and has the potential to greatly increase the number of young a pair can raise in a year, but the success of this strategy is limited by the abundance of nest predators (since two nests need to survive through the nesting cycle) and food availability (since enough food must be available late in the season for birds to feed another nest full of nestlings).

Learning module developed by K. Langin, H. Sofaer and S. Sillett for the Hubbard Brook Research Foundation (2009).