Wednesday, July 22, 2015

First Year Bird Mortality

Note: This program first aired on July 18, 2015.

Today I have an update to the story I told last week. If you missed it, my niece and I recalled the experience of finding a bird nest that had been blown out of a tree. It was on the ground, and around it were the baby birds that had been in it. We put the birds back in the nest and put the nest back in the tree the best we could, hoping the parents, who were anxiously flitting around the tree chirping, would be able to resume their parenting duties. We realized that this was the right thing to do, even though emotionally it was really hard, our nurturing drive kicked into over drive and we just wanted to take the babies home to “save” them.  And I wish I could give you a happy ending to last week’s story, but the fact is, I can’t. When I went back the next day, the nest was empty, the babies gone, and the parents, if they were around, were silent. When my niece called later for an update, I had to tell her the truth. I was anticipating some melodrama, but instead she said so matter of factly “I kind of knew that was going to happen”, and that was the end of it.

You may wonder, as I have, if putting the babies back into the nest and leaving them was indeed the “right thing to do”. After all, even though we did the right thing, we left them with their parents, it didn’t work out. The reality is that even though they had the best chance of surviving under the care of their natural parents, that doesn’t mean that chance was very high to begin with. This is another one of those dirty secrets of nature that most of us never think about. Most things that are born don’t make it out of childhood. The average mortality for first year birds can be as high as 90%. For typical passerines or perching birds, juvenile mortality in the nest is around 50%, that is before the young even fledge. Nest predation and weather related accidents account for most of this mortality. Once the young fledge and eventually learn to take care of themselves, mortality pressure doesn’t let up but it changes form some what. Young birds aren’t as successful at feeding themselves, so starvation gets added to predation as another factor in first year bird mortality. Most of us, watching our back yard bird feeders, never have any idea the odds are so poor.

But what is the alternative? If a pair of robins in your yard has two successful broods a year (Robins are thrushes and will have a second and even occasionally a third brood in a single season if time allows), and each brood consists of 4 young, at the end of the season where there were 2 robins we now have 10. An environment that easily supported 2 robins may not so easily support 10. And what of next season, if all 10 of those robins were to survive and reproduce? The world would soon be flooded with robins, the environment denuded of all appropriate robin forage. It isn’t even a realistic scenario, and we all understand that. But in order to have the realistic scenario, a relatively steady state of robin population in our back yards, young robins die. Old ones do too, but robins are adapted to have more babies hatch than old birds dying, because the young are such easy prey. If you have many offspring, hopefully one will make it to old age.

Humans used to live this way too. When child mortality was much higher than it is now, women gave birth to many more children. As health care has improved and child mortality has dropped dramatically in most parts of the world, the number of children a woman gives birth to has dropped as well. We no longer have to hedge our bets like the birds do.

Most birds will try again and renest when they experience a nest failure, like those chipping sparrows at my neighbor’s house. There’s still a chance that we will see their young flying this summer, just not the young from that ill fated nest. And when I see an adult bird, I’ll see all of its nestmates now as well, the ones who pulled nature’s short straws, enabling life as we know it, to go on.


Uncited but with interesting math:

Uncited but good ideas (personal blog of a bird researcher)

The free shelf at my local academic library yielded a 1975 copy of Wallace and Mahan’s An Introduction to Ornithology 3rd Ed, which has been a great source of foundational material!