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).