Pleistocene Cowbirds

Nice illustration of male and female cowbirds.  During the Pleistocene there were twice as many species of cowbirds as there are today.  Cowbirds require fragmented habitat with a mixture of woodland for nest parasitism and grassland for foraging.

70% of large mammal species became extinct at the end of the Pleistocene, but very few birds did–evidence supporting overkill models of extinction over those of climate-induced ecological change.  Of the few species of birds that did become extinct, almost all were commensal species dependent on the existence of abundant populations of large mammals.  Most of the extinct Pleistocene birds were condors, vultures, and eagles–the scavengers that relied on carcasses of mammoths, mastodons, and other megafauna.  But there were also 2 species of cowbirds that depended on large herds of grazing animals, and they couldn’t survive the North American extinctions of the grass-eating mammoths and horses.

Cowbirds are in the blackbird family.  There are 2 extant and 2 now extinct species.  The brown-headed cowbird (Molothrus ater) occurs all across the continent, while the bronzed cowbird (Molothrus aeneus) is restricted to the southwest.  Fossils of the Pleistocene cowbird (Pandaneris convexa) have been found at the La brea tar pits in California, at least 2 sites in Florida, and 1 in Virginia, so evidentally it was widespread. Pyelorhampus sp., another Pleistocene species, left fossil evidence in New Mexico.  Its geographic range is unknown.  Fossil evidence of the 2 extant cowbirds has also been found in Pleistocene fossil sites.  Cowbirds abounded in the world of the Pleistocene because ecological conditions were ideal for their lifestyle which is a fascinating one.

During spring and early summer cowbirds spend early morning hours hunting woodlands for other bird’s nests.  When they find a nest filled with eggs, they carefully wait for the parent to leave.  As soon as they have a chance, the cowbird will lay her egg on the other bird’s nest and abandon it to be cared for by the unwitting other bird.  This is known as obligate brood parasitism.  Non-obligate parasites are birds that parasitize nests of their own species.  The cowbird is an indeterminate egg layer capable of laying 30-40 eggs over a 2-3 month period.  That equates to at least 30 parasitized nests per cowbird.  No wonder they’re so plentiful.  The cowbird chick usually hatches first and is so much bigger than the other nestlings that they outcompete them for food and may even shove them out of the nest.

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Cowbird chick in the nest of another bird species.  Note it’s about 4 times bigger than the host nestlings. Few nestlings survive the competition with the bigger chick.

Very few nestlings ever survive a parasitized nest. Cowbirds are known to successfully parasitize 144 out of 220 bird species, but many birds have developed successful defensive strategies.  Shrub nesting birds such as kingbirds and mockingbirds mob and drive off cowbirds before they can lay their eggs.  Birds that live in deep forest are more naive than shrub-nesting birds, but some learn to recognize a cowbird egg and will knock it out of the nest.  Others build a new nest over the old one and lay a new clutch of eggs.  One report mentions a 5 story nest built over repeated parasitic attempts.  And a few species simply abandon the nest and build a new one elsewhere.  Some parasitized nests are doomed to failure because the diet of the host species is too dissimilar.  Cowbird nestlings can’t eat food brought by gulls and hawks because it’s too meat-centric, while that brought by house finches is all seeds and not enough insect.  Nevertheless, cowbirds are so successful that in 1 year they replace 100% of numbers killed in biological control programs meant to protect endangered species such as Kirtland’s warbler.

Efforts to control cowbird populations are probably misguided.  Anthropogenic habitat destruction is the cause of every single listing of a bird on the endangered species list.  Cowbirds are only part of the natural mortality these birds have evolved with for millions of years.  Some ecologists fret about the habitat fragmentation caused by modern development and how this creates habitat favorable to cowbirds but detrimental to deep forest species such as vireos and warblers.  However, Pleistocene environments in North America south of the ice sheet are interpeted through pollen records as having been a patchy fragmented mixture of woodland and grassland–the ideal habitat for cowbirds.

After spending spring and early summer mornings searching though the woods for a nest to parasitize, the females fly to the grasslands and join the males.  They closely follow herds of bison (formerly) and cows (more commonly since the 19th century) because they prefer short-grazed grass over tall grass.  Although insect diversity is higher in tall grass, grasshopper and leafhopper density increases in short-grazed grass, providing the birds with a banquet.  The shorter grass also gives the cowbirds greater visibility, so they can avoid predation.  The cows themselves give the cowbirds a place to perch where they can get an even better view of possible danger.

Brown-headed cowbirds survived the end of Pleistocene extinction because of their close attachment to bison.  Pandaneris convex may have had a closer attachment to mammoths and/or horses–the 2 other great grass-eating beasts of the Pleistocene.  And when they became extinct in North America so did this species of cowbird.  Perhaps, the paleoindians even saw them perched on the backs of mammoths and knew them as mammothbirds.

In late fall and winter different species of blackbirds (cowbirds, red-winged, rusty, grackles) flock together.  They are the only large flocks of birds I ever see.  Most other flocking birds have been decimated and are rarely seen in the numbers of yesteryear.

Reference:

Morris, Dana; and Frank Thompson

“Effects of Habitat and Invertebrate Density on Abundance and Foraging Behavior in Brown-headed Cowbirds”

The Auk 115 (2) 1998 376-385

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