Saturday, July 25, 2015

The Principles of Natural Selection as they apply to Baby Birds

Note: This show first aired July 25, 2015.
Natural selection is the mechanism by which evolution takes place, and by evolution, I mean the change in genetic material in a population over time. That change over time is thought to make populations more robust, more “fit”, fitness meaning more successful reproductively; and successful reproduction is the name of the game when it comes to life on Earth.

Darwin’s theory of evolution by natural selection is based on three simple premises: that more individuals are produced than can survive, that there is variation in the traits of those individuals, and that the individuals with the traits best suited for survival and reproduction in the environment will survive. The genes that code for the successful traits get passed on. Genes that code for less successful traits get passed on far less frequently, and eventually disappear from the gene pool. That is how over time, the genetic makeup of a population changes.

We’ve been talking the last couple of weeks about an ill fated pair of baby birds, they were discovered tumbled out of a nest that was blown out of a tree, and birds and nest were replaced into said tree as best an anxious adult and curious child could do. Upon first check up, the baby birds were discovered to be out of said nest again, and replaced a second time. Upon second check up, both babies were gone, as were the parents. In some ways the experience feels like failure, but as I talked about last week, the odds for those babies were never very good even in the best circumstances, which brings us to that first tenet of Darwinian evolution: more individuals are produced each generation than can survive. So why didn’t these birds survive? 

The second tenet of natural selection states that there is variation in traits among individuals in a population. Among all of the chipping sparrows in the northeast, some make very strong well protected nests, others make flimsier ones. Some choose nest sites out of the prevailing wind, others choose more exposed shrubbery. Nest building is a very instinctual behavior in birds (though there are studies that show that as some birds practice, their nests get better, indicating that there is a significant learning component to nest construction as well). The instinctual part of nest building results from genetics, some birds have genes that code for proteins that end up directing cells to do X, other birds may have genes that code for proteins that direct cells to do Y. If X causes the nest to be built in a very secure location, and Y causes the nest to be built in a more exposed location the genes have significant implications for the fitness of the individuals. Those birds may survive, but where they locate their nests impacts the survivorship of their young, and whether or not the genes of the parents get passed on into the chipping sparrow gene pool or not. If they can’t successfully reproduce because they keep building their nest in an unsuitable place, they are a genetic dead end, which brings us to Darwin’s third tenet of natural selection. Genes that code for successful traits get passed on, genes that code for less successful traits eventually die out.

There is one more factor beyond how successful or not the traits your genes code for are, and that factor is simple bad luck. Sometimes organisms are doing all the right things, have robust traits, and high survivorship, when bad luck strikes. In the case of birds, they could be in a tree that gets cut down, or in a tree that gets damaged by lightening or a falling nearby tree, things that don’t have anything to do with their nest location and building skills. And that is the other hard truth about natural selection, there is a large and strong random component to it. Just as the mutations that give the gene pool variation are random, so are the incidents of “bad luck” that can change the distribution of genes in a population without respect to their impact on fitness. These shifts in gene frequency due to bad luck (or being in the wrong place at the wrong time) are called genetic drift, and the smaller the population size, the bigger an impact it can have. 

The mechanism of natural selection cuts a broad and unforgiving swath through populations every reproductive cycle, and while there can be bad luck, there is rarely good luck. The modern human experience of the world is so different from this that it is hard for us to remember. In nature, the default is failure. Keep this in mind as you look around you and recognize what you see for what they are-the chosen few, the rare success stories amidst nearly infinite attempts at life.


On birds, nests, instinct and learning—this well publicized study came out in 2011 and challenged the long held assumption that bird nest building is entirely instinctual

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!

Wednesday, July 15, 2015

What do do with a baby bird

Note: This program first aired on July 11, 2015.

This week's show featured my niece Caitlin as we described the experience of finding a bird nest on the ground one windy day, and then finding the baby birds that went with it. We talked about what we did and how it felt, and how doing the "right thing" was hard.

The internet is a great resource for leaning about what to do in that situation, if you go to a reputable site. The internet can also trip you up (as Caitlin said to me that night "the internet lies!"), so you have to be careful about who you listen to.

Some of the sites I used to figure out what to do are listed here:

There are lots of Wildlife Rehabilitators in Maine, but they don't all take birds (that requires a federal permit. If their listing says they don't have a permit, that means they can't take birds).

The most well known bird rehab facility in Maine is Avian Haven in Freedom (Waldo County).

Remember, all migratory non game birds are protected by federal law. It is against the law to possess any bird or bird part (living or dead) without a federal permit!

Dance of the Sand Worms

Note: This program first aired July 4, 2015.

A few weeks ago, I got a call from my nearly 4 year old nephew during dinner. I could hear the excitement in his voice as he shouted into the phone about seeing worms in the water at the town dock. I asked him what color they were and he said they were all colors. This brought back a memory from my own childhood, a memory until this moment I wasn’t even sure was real. I remember seeing huge iridescent blue green worms swimming at the surface of the water, and then seeing them washed up dead on the beach the next day. I saw them that one time, and never again, causing me to think I had either imagined it or witnessed something very rare. Hearing my nephew’s excitement a couple fo things became clear, what I saw was indeed real, and that now my family has an official childhood rite of passage: witnessing the spawning swim of the sand worms.

Sand worm, rag worm, clam worm, these common names all refer to various species of polychaete worms found in coastal areas throughout the north Atlantic. Polychaetes are marine segmented worms, meaning their bodies are divided up into hundreds of discrete segments. Believe it or not this body structure represented a big leap forward in terms of evolutionary complexity. Polychaete worms are an incredibly diverse and successful group of organisms, there are over 100 species listed for the Gulf of Maine alone.

The worms I saw as a child, and my nephew saw this spring are Alitta virens, sometimes known as the king rag worm, or sand worm. They live in burrows in low intertidal or sub tidal sand or mud. The burrows are U shaped, with each end of the U an opening  at the surface of the sand. The worms spend most of their lives in these burrows, pumping oxygenated water through them so they can breathe (and a neat side effect is the diffusion of oxygen into otherwise anoxic sediment. The oxygen burrowing worms draw down into the sediment fuels an entire microscopic ecosystem). They stick their fearsome heads and sharp black teeth out of the burrow to feed on various small benthic sea creatures, and are said to sometimes scavenge omnivorously. Rag worms are among the most long lived of polychaetes, some are thought to reach the ripe old age of as much as 7 years. In the end though, regardless of how old they get, their fate is the same. This group of polychaetes practices what is called “semelparity”, meaning they reproduce once, and then they die.

As I said, these worms spend their lives in their burrows, only occasionally leaving to crawl along the bottom to find better digs, otherwise they are in the sediment. Things change however, when it is time to mate. The males undergo epitoky, their bodies change shape and structure in preparation for mating. They turn shimmery blue green and learn to swim, and around the new moon, they swim to the surface at high tide and release their sperm into the water column. That sperm makes its way down to the females who have remained in their burrows. They fertilize the eggs they have spent nearly a year developing in their bodies. Once the eggs are fertilized, the female excretes them on the surface of the sediment outside her burrow, where they develop, hatch and start a new generation of sand worms. After shedding sperm, the males die. After laying the eggs, the females die. Such is the life of a sand worm.

The moon is involved because the worms, both male and female are highly sensitive to photoperiod, and the development of the eggs begins a year before mating in response to changes in day length. Something about the combination of warming water in the spring, lengthening days, and darkness at night tells the males that the females are ready, but as far as I can tell, we can’t know for sure when the magic will happen.  That is why it is such a good idea to be outside as much as you are able. You never know when you will stumble upon a certain flower blooming, a case of tiny spiders hatching, baby birds fledging, a lynx track in the snow, or hundreds of two foot long blue green worms swirling in the water doing their once in a life time dance. 


Everything you want to know about clam and blood worms:

All about Annelids, from the Tree of Life:

Actually, even more that you might want to know about polychaets, from and OLD Woods Hole guide:

 On lunar impact of breeding (but not of worms specifically)