Burke Museum
Burke Museum
4/26/2012 Birds- Sex, Treachery, and Death
Today at the Burke we learned about mating, population, child rearing, nesting possibilities, eggs, fighting, and gender differences in birds. It was cool to get to see the different eggs, plumage patterns, and changes in species over time. The lesson was divided into three separate parts- treachery, sex, and death.
Treachery: Laying eggs and raising young is a costly investment for birds. It takes time and energy, but it is worth it in order to pass on genes. Colonial nesters, or birds that all nest in the same tree, often have different egg colors and patterns. This is so that parents can identify if someone has added an egg to their nest. The weaver egg can be white, tan, turquoise, and may be speckled or not. This helps them to identify if an intruder was trying to reduce the amount of work in raising their young. But some birds have adapted an interesting way of shortcutting this process. The brown headed cowbird (BHC) doesn’t build a nest at all, instead it lays its eggs in other birds nests, often the yellow warbler. Yellow warbler eggs are much less varied than weavers, and it’s obvious when a BHC has added an egg, but they get away with it anyway. This is possible because BHC eggs are 30% thicker, so it’s hard for the little warbler to break the egg. The warbler does have a sharp beak though, so sometimes it is possible. But they have to be careful to not damage their own eggs in the process. A quarter of the time warblers try to pierce a BHC egg they damage their own in the process, so they have to decide if it’s worth the risk. That is, if the warbler was willing to risk doing such a thing. BHCs will attack, kill, and bully other birds into raising their young. If BHC discovers that their young was not raised or properly cared for they will sometimes come back and kill the babies in the nest. This kind of enforcement is called the mafia hypothesis. Sometimes warblers will stack nests up on top of each other and wait to lay their eggs because BHC’s will continue to fill up their nests. This process of a bird laying its eggs in another bird’s nest is called parasitizing. Once the young are hatched the male warbler often feeds the BHC chick. Females tend to spread the food around evenly, whereas the male often feed the biggest chick, which is the BHC. Studies show that BHC brains get bigger during egg lying season in order to keep track of all the nests they’ve parasitized.
Death: We can use museums to infer many things about bird population structure. It is important to know about population structure to make sure populations are stable. The ratio of juveniles to adult, birth rate, death rate, life expectancy, and migration can all help figure out how bird populations are doing. Population graphs can be helpful in many ways, but they have one major drawback: they can’t specify where changes or problems are coming from. One interesting fact I learned in this lesson is that seabirds are often white underneath, to disguise and camouflage themselves form underwater predators, and dark on top so they are harder for predators from above to see. I also learned that when Townsend and Hermit warblers migrate the juveniles often end up higher on the mountain. This is because the males are territorial and come back and claim the best space at the bottom, then the juveniles arrive later and have to find places at higher elevations.
Sex: Male birds are generally bigger and more colorful to attract mates and ward off competition. Being this way is costly for survival, but it’s worth it because it’s good for reproduction. This is known as sexual selection. There are a few exceptions when males are smaller and less colorful than the females. In these cases the male is in charge of parental care. Females will often establish a territory and then have multiple mates but only one father cares for all the young. This is known as cuckeling- when a bird raises someone else’s egg. If there is one male that mates with lots of females in a species that species will have smaller testes in proportion to their body than if there is lots of males and the female has her choice. Gorillas, for example, have one male in the group that mates with whoever they want- so they have very small testes. Female chimps, on the other hand, mate with many males so chimps have larger testes in proportion to their body. A random interesting fact that I learned in this part of the lesson is that male pheasants have a sharp, pointed spur for fighting over mates.
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