A View of Unconventional Birdlife
Obligatory nest parasites
Our two cowbirds species, the Brown-headed and the Bronzed, are the only birds in North America that are completely dependent on other birds to raise their young. Therefore they are described as obligatory nest parasites. There are other nest parasites that lay their eggs in other bird's nests and incubate the eggs and raise the young themselves. But cowbirds are the only birds that abandon their eggs after laying and leave it to the host birds to completely rear the cowbird offspring as their own. That's the difference between nest parasites and obligatory nest parasites.
Unfortunately, most of the host birds are unable to distinguish their eggs from a cowbird egg in their nest. A female cowbird will lay a single egg in the host nest. She may lay between six and forty eggs in as many nests - never more than one egg per nest. However, on rare occasion, multiple female cowbirds may lay eggs in a single nest. As many as eight cowbird eggs, all from different females, have been discovered in a single host nest. After incubating the eggs in the nest, the host birds rear all the newly hatched birds.
Successful parasitism depends upon the proper timing of the laying of the female's eggs. Females must keep several nests under observation simultaneously and at the appropriate times return to each nest to lay a single egg, usually one per day. So, aside from the production of her eggs (which some ornithologists speculate is aided by watching the host bird build its own nest), a female cowbird's chief contribution to reproduction is the task of finding suitable nests, then laying in each at the proper time - neither too soon, which may cause the builder to desert - not too late, which will handicap her progeny if they hatch.
Distinction of Eggs
Among the birds that can recognize the differences are the American Robin and the Gray Catbird. These birds will immediately push the cowbirds eggs out of the nest.
More often, however, another strategy is used by a larger number of host birds. These birds, as best exemplified by the Yellow Warbler, will simply build a new nest on top of the first nest, burying the eggs between the nests. Yellow Warbler nests have been discovered that are five levels high with the bottom four all containing cowbird eggs. The warblers are more insistent on having a pure brood than the cowbird is willing to repeatedly lay more eggs in the same nest.
Other species of birds employ another strategy to defeat the successful hatching of cowbirds eggs in their nest. Painted Buntings, Field Sparrows, Northern Cardinals and Yellow-breasted Chats most often abandon the nest with cowbird eggs in it and start over at another location. But, sadly, a great variety of birds accept the foreign eggs, hatch out young cowbirds that they raise as their own, usually at the price of some or all of their own offspring.
Hosts and Diminishing Populations
Bronzed Cowbirds are known for destroying the eggs of other cowbirds when they find them in host nests. Hosts for Bronzed Cowbirds frequently include flycatchers, wrens, thrushes, mockingbirds, thrashers, vireos, wood warblers, and tanagers, but chiefly a variety of sparrows and finches, and, above all, orioles and a few related birds. Brown-headed Cowbirds, by contrast, utilize a much wider variety of host species nests. Customers have shown us photographs they have taken showing large cowbird babies begging for food from "adoptive" Northern Cardinal and Pyrrhuloxia parents.
Cowbirds lay their eggs in over 220 other bird species nests. Most frequently, victimized birds are smaller than cowbirds and build their open cup-shaped nests in trees, bushes and on or near ground level. Cavity nesting birds; such as woodpeckers, bluebirds, and chickadees are rarely bothered by cowbirds.
Ornithologists now recognize that cowbirds are partly responsible for the demise and/or extinction of several species of birds, particularly some of our rarest warblers. The last remaining nests of the now extinct Bachman's Warbler were all found to have cowbird eggs in them and only the cowbird babies survived. The Kirtland Warbler is now facing extinction, with a total population of only a few hundred. Ornithologists have saved this species (at least for the time being) by physically trapping and removing cowbirds from the warblers only known breeding grounds - a now protected area in north-central Michigan, where controlled plantings and fires produce the warblers required habitat of thickets of young jack pine trees.
In California, the Bell's Vireo conservation status is increasingly threatened as a result of cowbird intervention. Black-capped Vireos throughout their range have also seen a sharp decline in their numbers as a result of cowbirds interfering with their normal breeding. Cowbirds definitely play a part in the demise of some of these endangered species but habitat destruction also plays a crucial role. These warblers traditionally spent their winters in the Caribbean Islands, but much of their winter territory has been developed into resorts and housing areas - so the cowbirds shouldn't be assigned all the blame. While cowbirds have been partly to blame in the demise of many songbird species, it should be made clear that predation (cats, hawks, snakes, etc.), physical stress of migrations, crashes into windows and buildings, habitat fragmentation, disease or illness, old age, etc. accounts for far more deaths than cowbird predation.
Fertile female cowbirds often stake out and survey smaller songbird nests. Once identified, the cowbird returns to that nest shortly after sunup once the parent bird goes off to forage. The host bird already has her eggs in the nest when the female cowbird deposits a single egg to the nest. Laying her egg in the host nest usually takes only seconds and the female quickly departs before she can be noticed. Quite often the cowbird simply devours the host's eggs, or in rare instances, pierce the egg in order to grab it and carry it off where it can eat it elsewhere and out of sight of the host bird's nest. Cowbirds are able to distinguish their eggs from the host's and only carry away or destroy the host eggs. By late summer, fledged cowbirds join up in flocks of their kind.
Range and vocalization
Cowbirds at our feeders