Monday, May 28, 2007

Nest success

This is the time of year when birds are building and using nests. But as parents, birds are often unsuccessful, both at building nests and at protecting their occupants.

Bad weather and faulty construction can lead to nest failure. But most bird nests are also subject to predators. Various studies of nest predation have found that between a third and a half of nests are attacked by other birds, small mammals, and reptiles such as snakes.

Percentages of predation can vary widely. A study of Hermit Thrushes in Arizona found that predators attacked 83% of the nests. A study in the same state of Black-headed Grosbeaks found predators attacked only 23% of the nests.

Dr. Steven W. Kress reported a study that found that of 100 Song Sparrow eggs, 74 hatched successfully and 52 eventually fledged. That’s a loss of nearly 50%.

Years ago, Dr. Arthur Allen of Cornell University estimated that less than 20% of all nests succeed in producing a complete set of new, mature birds. But as Allan and Helen Cruickshank point out, that’s nature’s checks and balances. “Should all of the birds’ eggs laid in North America in a single season not only hatch but the young mature,” they said, “the continent itself would be so crowded with birds that man himself would suffer acutely.”

3 comments:

John S. said...

I watched a pair of robins abandon a nest after a troupe of blue jays repeatedly raided it. The jays stole little chunks of it for their own nests. Sometimes engineering failures take their toll, too. I saw a squirrel nest disintegrate over my front yard, dumping three young onto my lawn. The mom gingerly picked up each little one and slunk off with them, to parts unknown.

Larry said...

I was trying to watch nesting Brown Thrashers today.-It's not easy-they are so secretive.Song Sparrows seem like a good species to get nesting data on as they are more cooperative.

=jack said...

Squirrel mothers have an advantage: teeth. They can pick up and haul off the victims of their poor engineering. Birds don't have the same ability, but they do guard their offspring where possible.