Spring often conjures images of verdant hillsides covered in wildflowers, peaceful scenes of young romping animals and weather coming in like a lion yet leaving like a lamb.

But now, as spring fades into early summer, the season reveals a fierce competition for survival.

In the past two weeks I have watched adult carp swim through schools of newly hatched fish, gulping as they go, and a scrubjay nab a red-tailed hawk fledgling and fly away with the adult hawk close behind. Neighborhood cats get plump on the dozens of baby rodents born each week, and a nearby coyote has found a larder in these fat cats.

Just a few days ago I watched a scrub-jay chasing a brownheaded cowbird (Molothrus ater) through the branches of a willow tree. Although the cowbird is a few inches smaller and no direct danger to the jay, the jay was visibly disturbed by the cowbird’s presence.

Then I remembered that cowbirds indeed are a threat. They are well-known as brood parasites. That is, instead of building nests of their own, female cowbirds search for nests of other species in which to lay their eggs. This forces the host bird to care for the cowbirds’ eggs and, eventually, their young.

It is commonly believed that this parasitic strategy came about from the cowbirds’ life on the open plains. Their range in the United States was once thought to be restricted to grasslands where herds of bison roamed. The birds followed the bison, feeding off of insects kicked up by their movement.

It would be a good breeding strategy for the adult to leave its eggs in the care of another so it could continue to wander with the herds without being tied to one spot.

The bison or the egg?

But more recent taxonomic research finds that cowbirds originated in South America, far from bison. Therefore it would seem it was their brood parasitism that made them suited to life on the Great Plains in the first place.

Over the last hundred years, cowbird abundance and possibly their range has expanded out of the prairies states, now reaching from coast to coast. The cutting of forests for agriculture and increased development for suburbs has created ideal habitats for cowbirds, which forage in fields and open spaces and then travel to woodland edges looking for host nests.

Cowbirds are not particular about which birds’ nests they will invade. This generalist behavior may cause population declines in some host species, but it does not affect the cowbirds’ reproduction, another factor in their successful expansion.

When cowbirds lay eggs in a nest where the host birds are the same size or larger than the young cowbirds, there is little effect on the survival of the host fledglings. The same is not true if the host birds are smaller. The larger young cowbirds will out-compete the smaller hatchlings, which then starve and die before fledging.

However, both large and small host species are at risk when mother cowbirds lay their eggs. Often the female cowbird will peck and remove the host eggs from the nest, ensuring that her eggs are those primarily cared for.

Because of this brood parasitism, cowbirds are thought to be responsible for the decline of many songbird populations. While this may be a contributing factor, it might not be the primary cause of the decline. For the same reasons that the cowbird has become successful—expansion of open space and loss of woodlands— small passerines are losing ground due to loss of habitat to human development.

Born to survive

But regardless of human impact, cowbirds are designed for success. Being part of the Icteridae family, along with orioles, meadowlarks and other black birds, cowbirds are well-adapted to a generalist lifestyle.

Icterids have strong bills and musculature which allow them to open their bills wide with great force. This behavior, called gaping, permits cowbirds to insert their bill into a substrate and create a hole in which they can reach food otherwise hidden to other passerines.

In addition icterids have strong thick legs for walking on the ground, ideal for foraging after insects.

The male cowbird can be recognized by its glossy, black-green body and brown head, while the female is a lighter brown with a finely streaked breast.

About a month ago in Irvine I walked through a park working to restore the chaparral and riparian habitat. Surrounding the park were condos with planted trees, grassy lawns and parking lots, all suitable habitats for cowbirds.

At the end of the trail sat a large cage filled with wild cowbirds; a research project was under way. Someone was attempting to capture and remove breeding cowbirds.

As I walked away I wondered if it wouldn’t be better to cage human plans for development rather than the cowbirds.

Meghan Walla-Murphy can be reached at the following e-mail: mwallamurphy@yahoo.com.