Blue Grosbeaks are large buntings of forest edge with a widespread breeding range in
North America, Mexico and Central America. The males are often seen singing from
roadside wires and tree tops. While the breeding biology of this bird is likely similar to
its relative, the Indigo Bunting, there is little detailed information about Blue Grosbeak
nesting ecology, courtship behavior and song structure available. In addition, very little
is known about the populations that breed in Mexico and Central America, and even less
about the birds that breed in North Carolina on their wintering grounds. The Blue
Grosbeaks breeding range overlaps that of the related Indigo Bunting, so ecological
competition with this species may help to keep its overall numbers low. Indeed, the
Blue Grosbeak resembles a jumbo version of its smaller relative, and can be mistaken
for it without careful observation.
Found throughout the state as a breeder, the Blue Grosbeak actually has some of its
greatest nesting densities in temperate North America in the Piedmont areas of the
Carolinas and Georgia. It is a neotropical migrant, although a few birds sometimes
linger in the Carolinas into winter. Neotropical (New World) migratory birds breed
during summer in temperate North America, migrating north each spring from wintering
areas, then fly back south to spend the bulk of the year in Mexico, Central or South
America, or the Caribbean.
This is a bird of forest edges and shrubby fields, and they are also found in old
logged-over areas, streamsides and hedgerows. At this time in our state there is a good
amount of this type of habitat, and is very possibly more common now than before
European settlement. They seem to thrive in disturbed sites and can even be found singing in
small brushy woodlots in quite urbanized areas, especially if water is nearby.
Breeding Bird Survey (BBS) data show an increase in numbers over the species range
between 1965 and 1979. Between 1982 and 1991, increases were indicated in states
along the northern edge of their range, such as Minnesota, Indiana and Ohio, but
decreases occurred in Pennsylvania, Missouri and Florida. Currently according to the
BBS this species seems to be increasing.
The Blue Grosbeak is sexually dimorphic (males and females look different). Adult
males are purplish blue with two brown wingbars. The female is brown with occasional
blue feathers on upper parts, and two brown wingbars. Young resemble the female, with
males having varying amounts of blue on their body.
These birds have stout conical bills, larger and stouter than that of the smaller
Bunting. Also, Indigo Buntings lack wingbars, except for young males, which are often
mistaken for adult Blue Grosbeaks. Female and young Brown-headed Cowbirds are
larger and lack wingbars, and the juvenile cowbirds have faint streaking below.
As brilliant as the males of this species are, only a few Blue Grosbeak nests have ever
been found. This is interesting in that these are birds of forest edge, transmission-line
corridors, open slashings left after logging, hedgerows, streamsides and even multi-age
pine forests. Females most likely build the nest, usually low in small trees, shrubs, or
tangles of vines, briars and other vegetation. Nests are often built near open areas or
even roads. The small bowl-shaped nest sometimes contains manmade items like rags,
cellophane, string or even newspaper. Females do all the incubating and most of the
feeding while young are still in the nest. The male feeds young more actively after they
fledge and while the female is building a second nest.
These birds are heavily parasitized by the Brown-headed Cowbird and also a known
host for the Bronzed Cowbird. Blue Grosbeaks have been observed building a nest on
top of a parasitized nest, and have been documented successfully raising both cowbirds
and their own young. Unlike some of the breeding neotropical migrants of eastern North
America, the Blue Grosbeak has evolved with cowbirds in parts of its range for some time.
At the end of the nesting season, Blue Grosbeaks form large flocks that feed in weed
and grain fields before flying to their wintering areas in Central America. During the
breeding season adults feed on a variety of insects but especially favor grasshoppers,
beetles, cicadas and mantis. Seeds of wild and cultivated grasses are also consumed, as
well as snails. Their large bill allows for effective manipulation of seeds as large as corn,
and large insects.
Courtship and nesting behavior is not well known, but males do arrive on the breeding
grounds before females, and it is assumed they are monogamous for the nesting season.
Males usually sing from high perches, and their song serves to attract a female and to
proclaim his territory to other males. The song is a long, rich warble that has a certain
huskiness to it. It lacks the paired notes of the Indigo Bunting song, and the burry
quality of the Purple and House Finches. The Orchard Oriole is similar but its song has
distinct phrases. The Blue Grosbeak also has a habit of flicking and spreading its tail,
even while singing.
Blue Grosbeaks breed throughout the lower half of the United States and into Mexico
and Central America, where they are year-round residents. They winter mainly in
Mexico and Middle America south to central Panama. Most of the Blue Grosbeaks
nesting in the eastern United States probably migrate across the Caribbean. They are
known to live almost six years.
There are no published information, but populations may be expected to benefit for the
abandonment of agricultural land in the breeding range. They seem to be widespread in
the Carolinas, but are not as frequently encountered as Indigo Buntings. The effect of
tropical agricultural practices and deforestation in the wintering range is unknown.
Suburbia does not seem to support many breeding Blue Grosbeaks, perhaps due to
predation problems with domestic and feral cats. Apparently little management is
needed, but the lack of basic information on this species makes it difficult to assess
management or conservation needs.
The Birders Handbook by Ehrlich, Dobkin and Wheye, 1988, Simon & Schuster
Eastern Birds by Farrand, 1988, McGraw-Hill Books