As we are all aware, the populations of migratory songbirds have
significantly declined in the past forty years. A study published in the Proceedings
of the National Academy of Sciences (PNAS) in December projects that
by 2100 up to 14 percent of all bird species may be extinct and that as many as
one out of four may be functionally extinct—that is, critically endangered or
extinct in the wild. These findings
come on the heels of the November 2004 Global Species Assessment by the World
Conservation Union (IUCN), which found that 12 percent of all bird species are
already threatened with extinction.
The PNAS study cited
several reasons for the current and expected decline in bird populations, including
habitat loss, disease, climate change, competition from introduced species and
exploitation for food or the pet trade.
One of the most precipitous declines has been documented in the Cerulean warbler population. According to the Breeding Bird Survey (BBS), there has been a decline of 4.5% per year from 1966 to 2001. Other statistics record a 79.6% loss from 1966 to 2003!
Of the many migrant birds that will be arriving in our region, one of the earlier migrants is the Brown-headed Cowbird (Molothrus ater). Male resident cowbirds arrive the last week of March, while resident females arrive first week of April. These birds arrive from their winter range in southeastern US, where up to 50 million have roosted, along with red-winged blackbirds, grackles and starlings before fanning out over North America.
The Brown-headed Cowbird has a unique breeding strategy. Instead of building their own nests, incubating their own eggs and raising their own nestlings, cowbird females use other bird species as hosts -- laying their eggs in the nests of other bird species and relying on these hosts to incubate and raise their chicks. In the long list of birds thus imposed upon, the vireos, the wood warblers, and the small sparrows figure most prominently. Scientists have now recorded that Brown-headed Cowbirds have parasitized over 220 host species. While not all hosts make good foster parents (timing of egg-laying, location of nests, size of eggs, etc.), cowbird chicks have been successfully reared by over 150 host species, with songbirds comprising the majority of hosts.
In recent decades, many land managers, conservationists and citizens have argued that parasitism by Brown-headed Cowbirds is a major threat to North American songbird populations and that cowbird parasitism is responsible for the range-wide population declines currently shown in a number of songbird species, particularly neotropical migrants. In response, various federal and local government non-game agencies have initiated programs to control cowbird numbers over the past 25 years.
So, what is the story with this ‘obligate brood parasite’? How did it evolve such a strategy? What is the impact of this bird on our resident bird population and, most significantly, what should we be doing about it?
Historically, during the last Pleistocene Ice Age, cowbirds probably followed herds of now-extinct species of North American elephants, horses, camels and pigs. In more recent history, they lived entirely in the Great Plains and followed migratory bison herds across the landscape, feeding on insects stirred up by the feet of the big ungulates. This transient lifestyle made it difficult to take up house-keeping in any one locale. Consequently, cowbirds developed a breeding strategy known as "brood parasitism," in which they would lay their eggs in the nest of another species, allowing this "host" species to raise the cowbird offspring. This strategy has worked well for cowbirds for thousands of years.
When bison were nearly extirpated from the North American landscape and replaced by cattle, sheep and goats, cowbirds adapted and began to associate with livestock. In the last century, Brown-headed Cowbirds have experienced massive range expansions and population explosions as forests have been opened to make way for agricultural and suburban landscapes.
The fecundity of the female cowbird is astounding. A single
female is capable of laying nearly one egg per day at the peak of the breeding
season, and produces a total of 30-40 eggs over the 2-3 month breeding period.
Because female cowbirds usually lay only one egg in a host nest, this
translates into 30-40 nests parasitized per female in one season.
In approximately 60-70%
of nests parasitized, the cowbird removes one of the host eggs before laying
her own egg. Normally, one egg per nest
is the rule, but two eggs are not uncommon (usually the result of two different
females), and three or more have been recorded in areas of high cowbird
density.
The cowbird has another
advantage that helps assure the success of her progeny. Whereas most host species require an
incubation period of 12 to 14 days, the cowbird egg requires only 11 to 12
days, thus enabling a head start on the host eggs.
Not surprisingly, the host birds react to the alien eggs in a number of ways. Nine species have the ability to reject/eject these foreign eggs from their nests, including Baltimore orioles, gray catbirds and American robins. Some occasionally cover over the parasite eggs by building a new floor over them if they have no eggs of their own at the time. This is true of such birds as the Red-eyed, Warbling, Blue-headed and Yellow-throated Vireos, the Prothonotary, Yellow and Chestnut-sided Warblers and the Redstart. However, the Yellow Warbler is the star performer. Two-story nests of this warbler are fairly common, where cowbirds are numerous; three-storied nests are not very rare, and as many as four or even five stories have been built. But in the majority of cases, the host birds accept the eggs and raise them along with their own eggs.
Why don’t more species reject the cowbird eggs? Perhaps some
accepter species, particularly those inhabiting (formerly unbroken) forests,
only encountered cowbird parasitism in the last 100 years and so have not
evolved to recognize and reject the parasitism. Or, it could be removing the cowbird egg could result in breakage
and loss of their own eggs, or raising the cowbird is a better option than the
risk of not nesting at all. Or, it
could be the impact on the host species isn’t significant enough to warrant
evolving more aggressive defensive tactics.
A major study was
published by Russell T. Norris. His
results included the following:
In the 237 observed nests, the hosts laid 668 eggs, of which 383 (57.3
percent) hatched; the Cowbirds laid 108 eggs, of which 46 (42.6 percent)
hatched; 37.7 percent of the host eggs, 26.8 percent of the Cowbird eggs
produced fledglings. Of the host eggs that hatched, 64 percent produced
fledglings; of the Cowbird eggs that hatched 63 percent produced fledglings.
With four exceptions all parasitized nests that produced young produced
at least one host young.
The 35 non-parasitized (successful) nests produced 2.94 fledglings per
nest; 19 parasitized (successful) nests fledged 2.05 host young per nest, indicating
that each parasite was raised at the expense of about one host young.
The most comprehensive findings have come from a national conference, Research and Management of the Brown-Headed Cowbird in Western and Eastern Landscapes, organized by Partners in Flight on 23-25 October 1997, Sacramento, California. (Details can be found at http://www.audubon.org/bird/research/.) The following is the summary highlights taken from the published ‘white paper’:
Cowbird populations are declining across the
continent (about 1% per year between 1966-1996).
Rates of nest parasitism vary locally:
when rates are high, parasitism may harm local populations of some species.
Host species often renest and are able
to make-up reproductive success lost to parasitism.
Hosts with short breeding periods and those that
begin the season raising cowbirds may not have enough time to renest.
Cowbird parasitism probably is not responsible for
the continent-wide declines of many North American songbird species.
Rates of parasitism depend on the
proximity of cowbird feeding sites to host breeding sites
Rates of parasitism sometimes correlate poorly
with numbers of cowbirds counted in an area.
Cowbirds are managed through lethal control.
Cowbird control is controversial.
Cowbird control is expensive.
Cowbird control programs have proceeded without a
general framework, with little coordination between programs, or between the
land management and scientific communities.
Cowbird control can reduce rates of parasitism
No research has tested the effectiveness of
large-scale control on the wintering grounds, where cowbirds congregate in
large roosts
Cowbird control programs on Endangered Species
have had some success in meeting their ultimate goal: increasing local host
populations.
Scientists suggest that cowbird control is a
short-term solution that ignores the real problem of habitat degradation as a
result of agriculture, grazing and development.
Scientists advocate protection and restoration of
host breeding habitat, and improvements in grazing and agricultural practices.
Cowbirds occur most often in agricultural/residential landscapes near
open woodlands. Cowbirds frequent woodland edges created when deforestation
leads to a mosaic of trees and open brush/grassland. They rarely occur near
continuous forests, deciduous or coniferous.
Large, contiguous forests sustain lower rates of parasitism than
fragmented forests. This is because cowbirds 1) scan for hosts at forest edges,
rarely in forests interiors; and 2) fragmented forests have proportionally more
edge than contiguous forests, creating small woodlots that are easy for cowbirds
to penetrate.
The most effective way to control cowbird numbers and the impact of cowbirds
on Neotropical migrants is to control the features of the environment on which
they thrive. Landscape-level measures that maintain large forest tracts with
minimal edge to interior ratios (i.e., compact versus long, linear shapes)
would be most effective. In addition, openings in forests such as for roads,
timbering, gas wells, picnic areas, etc. should be minimized and concentrated
into a few areas rather than dotted throughout the forest. Such long-term and
large-scale habitat planning is ultimately necessary for conservation of
Neotropical migratory bird species, not only to control cowbird populations but
also to control populations of nest predators (raccoons, blue jays, crows,
squirrels) that also increase in fragmented forests. Only through sound habitat
preservation, planning, and management can we stem the expansion of cowbird
populations and conserve our migratory birds.