Late spring and early summer is a time when fledglings leave the nest. These juvenile birds start to hang out on branches and on the ground as they practice and work towards learning to fly. The parents of the fledgling will stay nearby to monitor and to bring food. Sometimes though, the parent bringing the food isn’t the same species of bird as the baby bird. This type of behavior is known as interspecific.
What is interspecific feeding?
Interspecific feeding is when one species feeds the young of another. There are a few reasons why the adult birds of one species may feed a young bird of a different species.
Parental hormones
One reason is replacement feeding. A mother bird that recently lost her own brood will still have elevated levels of prolactin. This hormone stimulates caregiving behavior and may drive the bird to feed nearby chicks.
Mistaken identity
Sometimes it’s a case of mistaken identity. Young birds can look similar with their sparse feathers. Baby birds also tend to have bright yellow gape flanges around their beaks to attract the attention of the parents. Busy parents may inadvertently feed young other than their own if they get distracted by the wide-open mouths, fluttering wings, and loud begging of begging baby birds.
Brood parasitism
The most common reason is what is known as brood parasitism. This is when one bird species lays its eggs in the nest of another bird species. The unsuspecting bird then incubate the foreign eggs and feeds the chick after it hatches, often without recognizing that the bird isn’t one of their own.
In the Northern California area, one of the most common sightings of this relationship is between dark-eyed juncos and brown-headed cowbirds.
Brown-headed cowbirds are obligate brood parasites. This means the parents are as lazy as you get. Brown-headed cowbirds don’t build nests or bother to raise their own young. Instead, they lay their eggs in the active nests of other birds who then raise them.
Brown-headed cowbirds and dark-eyed juncos
Dark-eyed juncos are a common target because they don’t distinguish between their own young and cowbirds. The eggs of the cowbird and the junco are both beige with brown speckles. The foster parents then incubate and feed the cowbird chick as if it were their own.
A study done in the Allegheny mountains of Virginia found that 39% of of dark-eyed junco nests had at least one brown-headed cowbird egg in them. While parasitized nests had fewer junco eggs due to removal by the female cowbird, junco chicks that shared a nest with one cowbird chick usually grew about as well as those raised without cowbirds.
References
Wolf, L. (1987). Host-parasite interactions of Brown-headed Cowbirds and Dark-eyed Juncos in Virginia. The Wilson Bulletin, 338-350.
