Burke Museum 2
Daily Account: Burke Museum day 2
Thursday April 26, 2012
1:30-3:30pm
Today we went to the Burke museum for the second time to talk about birds. Through three different rotations we talked about reproduction, sexual selection, and the different factors that can change populations over time. Some of the conversations reminded me of what I’d learned and considered in an animal behavior class last year. We first talked about populations and the need for museums to keep records of the past for determining how a population is changing, for example whether there are many more young being born than there are older, or whether the population is currently stable, etc. We can use specimens to figure out which birds are in which locations and if and how they’re migrating over time. Susan talked about how in some migratory birds (I think we were talking about Townsend’s warblers), the adult males have to fly back before the females to claim nesting grounds, from which the females choose a few weeks later. What I didn’t realize was that the older males can fly back sooner because they have more body fat sooner, and therefore find the better nests lower on the mountain as opposed to the so-so nests of the younger birds, who are less likely to mate for this reason. Thus we can get a ratio of the ages of birds on the mountain. This is something we can only tell by keeping a record and history of the birds. In this way, we can take a snapshot in time to learn more about behavior patterns and ecology of a place over time.
Next we looked at Marbled Murlets (Brachyramphus marmoratus), and differentiated between the Juvenile and adult birds by the differences in their plumage. A question that came up was why are there two different colored males? The answer once known, seems obvious: it is because the birds were taken from different seasons. There were two different patterns, one of a light underside and a darker back, and the other of a male who was dark all over. The first was taken outside of mating season, whereas they acquired the second specimen during mating season. The reason for this is that during mating season the males spend time nesting in trees where they camouflage better if dark. In the off season, they spend much of their time on the water, where from the underside a light belly blends in with the surface, just as a dark back blends from above. I think this is the reasoning for penguin coloring as well.
Apparently, another bird, the robin is found all over the world, yet drastically different in size and other details. Someone noticed that Robins further North might have redder breasts, a hypothesis, still undetermined. We do know however, that they don’t all lay blue eggs.
Following this discussion, we talked to Seever (sp?) about Reproduction and Sexual selection. First things first: there are some downright beautiful birds, albeit also extremely conspicuous, such as peasants, and the majority of them are male. Yes, this conspicuousness is bad for survival, Seever says, but in terms of reproductive success, more matings = more offspring which weighs more on the scale of importance when it comes to getting your genes into the future if you’re a bird. Females on the other hand, are typically less conspicuous, because they are usually the ones in charge of parental care and need to stay out of sight to protect their young. Apparently, in most species the males don’t even come near the nest. In this structure of life, males also tend to be bigger, and I’m pretty sure it has to do with the fact that males need to fight of other males for females (sexual dimorpism).
In one particular species however, the Jesus Christ birds/Hassanas, the females tend to be bigger. As is the case for many shorebirds hawks, eagles, etc. These marsh-dwelling Hassanas are polyandrous, meaning the female has a harem of males she lays clutches for. In this species the females actually fight for territory, with very sharp carpel spines which I never would have known of had Seever not shown us an example. There are also high rates of predation in the tropics. Is this because there are land animals that have access to the nests too? I thought this was the same for seagulls, with fox predation. Is this a similar situation? Also in this species, the male incubates the eggs, and a clutch is limited to four eggs at a time, a quantity which we don’t seem to know the exact reason for at the moment.
According to Seever, there is also a lot of cuckolding going on in the Hassanas. This is because if a male has to wait in line for a clutch, he might defect to another female’s nest. However, the female he’s with wants his genes and to keep him for herself, so she gets the best genes possible, therefore to avoid defection, she offers the second in line male EPC. The consequences of this are ironic. The second-in-line male actually gets a better deal, because after the EPC, the clutch she lays for the first in line is partly genetically the second guys genes. Then, she’ll lay a whole new clutch for the second male to have all to himself. But the question is, is he really better off if there’s always another guy second in line? Is there always someone in line?
Next, we talked to Josh about eggs. Upon observation of several different clutches of eggs, we concluded a lot about the birds responsible for them. The first observation that came up was that none of the eggs in the case were completely round. I actually find this quite interesting, and I’m wondering if there’s an adaptive benefit to having non-rounded eggs. Are there any birds that lay round eggs? This may seem like a silly question, but I am curious. Anyway, the conversation quickly turned to birds called Weavers, most likely named this due to the round hanging nests they weave in the trees in Africa. Weavers are colonial nesters, unlike many other species. Another observation was that the eggs were different colors even though they were all from the same bird. Apparently, there is a selection for having a “signature” on your eggs because of the egg dumping phenomenon of females from the same species go into others’ nests and dump eggs, because it is a lot less energetically costly if you can have someone else raise your kids. On the other hand the babysitters aren’t too keen on the idea as far as natural selection goes, so having your own egg color means it’s easier to distinguish impostor eggs. Another species to use this clever behavior is the Brown-headed Cowbird (Molothrus ater).
Brown headed cowbirds apparently do not nest at all, using the nests of other species of birds to raise their young. They lay 40-50 eggs in one season, finding all kinds of hosts. Later when the young cowbirds have hatched and received all their nurture from the host, the cowbird parent returns to retrieve her nestling. The question is why do the hosts go with it? Even if the egg looks so different, and they have to feed a monster of a cowbird along with their young, why don’t they just chuck the egg out of the nest while they can? Well, sometimes they do, and most of the time they do notice the difference in the cowbird’s egg, however, some birds cannot tell or cannot manage to rid the nest of the egg. One reason they don’t get rid of the egg is that there’s a potential for it to be costly. If they destroy the wrong egg, or loose one of their own while pecking at it, it might make more sense adaptively to raise the extra young bird. While they can tell a difference, the males apparently have a harder time telling the eggs apart, and they are the ones spending a lot of time in the nest feeding the babies. Something else I found interesting was the red color of the inside of the baby cowbirds’ mouths. Ironically, it may act as a super stimulus for the hurrying host bird to feed it faster, along with the fact that the cowbird infants are usually larger and this usually corresponds to reproducing more and without parasitism this is the bird to devote time and energy to. Some birds beaks can’t puncture a cowbird egg, so the size of the host and its beak matters as well. Josh showed us one video of a bird using her talons to get rid of the egg, but this is not a common occurrence. There is definitely an “evolutionary lag” going on for the hosts, and they may catch up eventually and find a way to better adapt, though it is quite complex what the cowbirds have going on. They even keep track of where they’ve laid eggs, thus can check in on them and throw out hosts young if theirs isn’t there (the mafia phenomenon). According to Josh, these birds have lost the ability to nest. My question was about evolutionary lag, and will the other birds will eventually develop a way to evolutionarily “outsmart” the Brown-headed Cowbird? I guess we’ll have to wait and see.
Species List:
Marbled Murlets (Brachyramphus marmoratus)
Brown-headed Cowbird (Molothrus ater).
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