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.
References:
Uncited but with interesting math: http://www.countrysideinfo.co.uk/bird_lifespan.htm
Uncited but good ideas (personal blog of a bird researcher) http://toughlittlebirds.com/2013/07/10/the-fledgling-problem/
Check out her entire blog, really nice stuff: http://toughlittlebirds.com/2014/05/30/helping-baby-wild-animals/#more-2653
The Junco study: http://www.jstor.org/stable/5000?seq=1#page_scan_tab_contents
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!
