Why these birds avoid parental responsibilities by dropping their eggs into another's nest

Birds are fascinating animals and after naturalist Brian Keating noticed the bird nesting in his backyard had extra eggs, it got him thinking about something called brood parasitism.

Essentially, that means that a bird will dump its eggs into the nest of another bird — relying on them to care for its young.

And it's more common than you might think.

Keating says, according to his spy camera, the flicker nesting in his backyard has 11 eggs rather than eight, which is pretty unusual.

"Only six have hatched, which led me down the pathway of thinking that it was probably another flicker who dumped her eggs in there for whatever reason," he told The Homestretch.

"It brought me onto this concept of why other birds drop eggs into other birds nests."

In Alberta, Keating says the brown-headed cowbird is known for doing this.

"The cowbird evolved to follow the great bison herds that once roamed our landscapes here in the prairies," he said. "So it never had a home base to raise its own chicks because it always had to stay with the herd."

He says the cowbird's parasitic egg-dumping behaviour was so successful, that they are known to do it to 220 different host species of birds.

And the population has continued to expand across North America, with the cowbird now roaming on the backs of cows.

"A few days ago, I was down in Waterton National Park and I was watching a bison herd in their paddock down there and again, there was another flock of cowbirds doing what cowbirds should be doing." he said.

Keating says the cowbird eggs and chicks are pretty easy to distinguish since the incubation time is shorter and the chicks grow faster than others.

And despite them growing up with a foreign parent, they still "innately" understand how to be a cowbird as they grow.

"They are attracted to members of their own species so they don't grow up thinking they're are a song sparrow. As soon as they finish their freeloading opportunity, they then join flocks of other brown-headed cowbirds."

Anti-cowbird strategies

But for the hosts, it can be costly to their own species.

"They sometimes can and do fight back," he said.

"I saw a little YouTube video clip yesterday of a bluebird beating a female cowbird to death when the bluebird returned to its nest and found it sitting on its eggs."

He says some species have even developed anti-cowbird strategies. For example, the yellow warbler will often bury the foreign egg under nest material so it dies, while the blue-grey gnatcatcher will abandon its nest and all of the eggs in it.

Despite these strategies, Keating says cowbirds are still winning the game.

"A single cowbird will lay about 40 eggs in dozens of different nests," he said.

And if the cowbird finds out that the host bird removed their egg, Keating says a study found it will return to "ransack" the nest 56 per cent of the time.

"They seem to periodically check on the nests that they've dumped eggs to after they've deposited those eggs in that nest," he said.

As well, if the host bird re-lays its own eggs, the cowbird will return to the same nest and lay its eggs almost 85 per cent of the time.

"So really, the cowbird is is the host's worst nightmare. And this may be an act of punishment or an attempt to force a host bird to produce a new clutch," he said.

While the cowbird commonly does this, it's also a proven strategy in 100 other bird species, says Keating.

"It's a program of surveillance where the parasitizing bird has to watch and see what's happening. It's deception and it's aggression. Nature is a fascinating place."

For more on Alberta's wildlife from naturalist Brian Keating, visit his website and check out these stories:

With files from The Homestretch.