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.”
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.
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.
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.
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