Why do some birds mimic the sounds of other species?

Although some birds learn their species’ song during their first year of life, others, including mockingbirds, continue adding to their repertoire as they grow older. Northern Mockingbirds can learn as many as 200 songs, and often mimic sounds in their environment including other birds, car alarms, and creaky gates. One theory is that if a female prefers males who sing more songs, a male can top his rivals by quickly adding to his repertoire some of the sounds around him. Possessing a diverse assortment of songs may indicate he is an older male with proven longevity and survival skills–good traits to pass on to offspring. An older male may also be more experienced in raising young or may have access to better resources. According to one study on the Edwards Plateau in Texas, mockingbirds with the largest repertoires have the best territories, laden with foods such as insects, wild grapes, and persimmons.

Some researchers have suggested that mockingbirds may use other species’ songs to warn those species to keep away from their territories, but this possibility has never been thoroughly investigated.

In North America, the Northern Mockingbird is perhaps the best known mimic, but renowned mimics, such as the lyrebirds of Australia and the Lawrence’s Thrush of South America, occur on other continents too. Male Marsh Warblers learn the sounds of other species on their wintering grounds in Africa. Perhaps these varied sounds impress potential mates when they return to breed in Europe. Indigobirds in Africa are also mimics, but for an entirely different reason. Indigobirds are brood parasites that lay their eggs in the nests of other species. For example, the Village Indigobird lays its eggs in the nest of the Red-billed Firefinch. Young indigobirds learn the begging calls of the firefinches that raise them so they will not be recognized as an intruder. Young male indigobirds also mimic their hosts.

The female Thick-billed Euphonia is a Neotropical bird that imitates the alarm calls of other species when her nest is threatened. These sounds may get the attention of other species to help in the attack of a predator or other perceived threat.

Some species not typically thought of as mimics also occasionally learn the vocalizations of other species. Blue Jays imitate the calls of Red-tailed, Red-shouldered, and Broad-winged hawks, for example. The function of these imitations is unknown.

In some cases, mimicry may result from the song-learning process gone awry, such as reports of a Vesper Sparrow and House Wrens singing songs of the Bewick’s Wren, and an Indigo Bunting and a Common Yellowthroat singing a Chestnut-sided Warbler song. It seems that a fairly large number of these occasional mimics are unpaired, suggesting that males who learn the wrong songs often fail to pass their genes to the next generation. Selection against birds who learn the wrong songs may thus be very strong, so “mistakes” are not perpetuated.