[Genus] | Molothrus
The molothrus genus, its members known as cowbirds, is placed within the Icteridae family and all of the species it contains originate from the new world. The genus was first introduced in 1832 by some guy called William John Swainson. The name molothrus comes from Ancient Greek, the first part meaning “struggle” or “battle” and the second part translating into “impregnate” or “sire”.
There are a total of seven recorded species of cowbird, of which six are still extant. The one that went extinct likely did so as a result of large mammals that they depended on for food disappearing, around 12 500 years ago.
Most members of this genus occur in South America, or southern parts of North America, with brown-headed cowbirds having the northernmost distribution, reaching up into Canada. Most cowbirds occur in generally more open regions, with only the shiny cowbird frequenting deep forest.
All cowbirds are roughly the same size, reaching lengths of 14 to 24 centimetres, wingspans of 35 centimetres on average and weighs of somewhere in between 30 to 50 grams, with males being significantly heavier than females. Cowbirds can be black, green, blue, purple and brown, and usually have iridescent plumage.
They heavily feed on insects, usually relying on cattle to do so properly. This is also the likely cause for them having evolved to be brood parasites, as laying their eggs in the nest of another bird allows them to stay with a herd, without wasting valuable time on incubating an egg and raising a chick.
As just mentioned, cowbirds are brood parasites. They lay their eggs in the nests of a variety of other birds, which mostly consist of other passerines, but they, specifically brown-headed cowbirds, have been recorded in up to 220 host species, including hummingbirds and raptors. 140 of those 220 have ‘successfully’ raised cowbird chicks. Each season a female can lay up to 36 eggs, the raising of which usually happens at the expense of the chicks of the host species.
Many cowbirds don’t abandon their eggs entirely after laying them, and instead periodically check on them and their young. When the host species removes the egg, in more than 1 out of 2 cases the cowbird returns and lays a new egg, which appears to have a greater success rate than the first one.
Most cowbird species aren’t threatened and are doing rather well, with the populations all being close to stable. Cowbirds, being dependent on three different kinds of animals (cattle, insects, and bird-hosts) may or may not be more vulnerable to damage than other animals, but as 5 out of 6 members of the genus were placed in the least concern category by the IUCN, there doesn’t seem to be any major threat as of now.
Funfacts:
→ Relationships are difficult to figure out with birds that don’t make nests, but cowbirds don’t seem to be monogamous, with both males and females having several mates during breeding season.
→ There has been a case recorded where 17 bronzed cowbird eggs were found in the same nest.
→ Some of the cowbird’s hosts, such as yellow warblers, are capable of recognising the eggs, but are too weak to do anything about them, and instead build a new nest on top of the old one, hoping that the cowbird won’t return to lay a new egg. Most host species do not recognise the eggs.
→ Cowbird eggs hatch much faster than other eggs, which is crucial in the young’s survival. They either have the benefit of gaining more food earlier than their foster siblings or get rid of the latter by tossing out the other eggs.
→ Bronzed cowbirds most commonly parasitise the nests of orioles, and the calls of those can actively attract both male and female cowbirds.
- Photo credits: JMC Nature Photos, Andrej Chudý, Rosa Gamboias, Priscilla Burcher, Greg Lavaty
