Cowbirds are unknowingly raised by ‘foster’ parent birds

By | April 11, 2017

If we were bison, we’d probably have a much higher opinion of brown-headed cowbirds.

You’d be hard-pressed to find a birding enthusiast – or anyone else, for that matter – that holds much love for the brown-headed cowbird. This bird, which is common throughout Missouri and many other parts of the U.S., has earned the ire of many people because it parasitizes nests of other birds.

Mid-April is usually when female cowbirds begin plopping their eggs into nests of various species. Although there’s little appreciation for these nest-infiltration tactics, even the most ardent cowbird-hater would have to admit it’s a rather remarkable method of producing the next generation.

The brown-headed cowbird (Molothrus ater) is in the Icteridae family, which means it’s a close relative of blackbirds and orioles. The male is glossy black with a chocolate-brown head; the female is grayish-brown and has few distinguishing marks.

Cowbirds can be seen foraging for insects in yards, parks and pastures, but the story behind their egg-laying method dates to pre-settlement times and this brings us back to bison. It’s theorized cowbird nest parasitism is a remnant of when these birds followed large herds of bison.

Cowbirds ate insects stirred up by bison and also sat atop these large herbivores picking off ticks and insects that crawled across their bodies. Unable to move with the wandering herds and also maintain and care for a nest of offspring, cowbirds developed the habit of laying eggs in other nests and relying on those birds to raise their young.

As large herds of cattle replaced bison on the landscape, these birds found insect meals on and around grazing livestock and – in the process – earned the name “cowbirds.”

Cowbirds were originally residents of the Great Plains, but they have expanded their range as a result of human development that fragmented forested areas. Cowbirds no longer follow large herds of shaggy bison or longhorn cattle, but they still use the reproductive method that benefited them back then.

Cowbirds are brood parasites – a term used to describe animals that rely on others to raise their young. There are several examples of brood parasitism in the animal kingdom, but in the case of the brown-headed cowbird, it involves an observant female that either finds other birds actively laying eggs or comes across a nest with eggs in it.

Once the female finds a host, she will sneak onto the resident bird’s nest when the parent is away and lay her own egg. Sometimes cowbird females damage one or more eggs in the nest to make room for their egg and reduce competition when their nestling hatches.

The “foster” parent bird then unknowingly raises the young cowbirds, often at the expense of her true offspring. Cowbird eggs require a shorter incubation period (10-13 days) than most other songbirds and, thus, hatch first.

Cowbird nestlings also are often larger and grow quickly. These advantages allow them to get them to get the bulk of the food provided by the parent birds, which result in reduced nesting success of the host species.

This method of reproduction has definite advantages for cowbirds. Because they’re not tied to a nest, they can continue to be active and forage throughout the day. This lifestyle leads to higher egg production – female cowbirds can lay more than 40 eggs per year.

One of the mysteries of this process is, since cowbird young are raised by other species, how do cowbirds learn to be cowbirds? Why don’t some cowbirds sing like cardinals and others chatter like blue jays. How do offspring develop the same vocalizations and mannerisms their biological parents have, even though they were raised by other species?

The answer is more of an observation than an answer – for reasons not completely understood, cowbird offspring don’t imprint on their adoptive parents like the bird’s true offspring does.

One theory is that female cowbirds stay in the vicinity of the nest and periodically call to their offspring. It’s thought this vocal identification may help young cowbirds find other cowbirds.

Approximately 200 birds have been documented as being hosts of cowbird young. Although this parasitism is obviously harmful to clutches of individual birds, there is still discussion about how much overall harm is suffered by some bird species.

In the cases of federally endangered species like Kirtland’s warblers and black-capped vireos, there’s no doubt cowbird parasitism has added problems to an already troubled situation.

With other species, the damage assessment is harder to make, particularly in cases where habitat destruction and alteration is taking place (along with nest parasitism).

Information about cowbirds and other birds that reside in Missouri can be found at mdc.mo.gov.

By Francis Skalicky

(Francis Skalicky works for the Missouri Department of Conservation in southwest Missouri. He can be reached at 417-895-6880.)

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