4/26/12 Return to the Burke Museum: Sex, Death, and Treachery

Today we discussed the behavior of birds in terms of sex, death, and treachery and evaluated why these behaviors are important in terms of understanding population dynamics.
I first discussed treachery and the way in which certain bird species act to benefit their own species in spite of the health and well-being of another. In this section, the main topic of discussion was social parasitism among bird species. On this topic we primarily discussed how certain birds use various tactics to minimize the effort they have to put into raising their own young. Some bird species have evolved over time to that the color egg-shell they produce is most similar to another species while others, like the Brown-headed cow bird use brute force to ensure that another bird will raise their young. Brown-headed cowbirds do not make nests and instead parasitize the nests of species who cannot effectively prevent such parasitism. Marsh wrens, for example, never have parasitized nests because they have extremely sharp beaks that can pierce eggs and kill invasive young in that way.
In terms of explaining this behavior exhibited by the Brown-headed cowbird, scientists have developed the Mafia hypothesis. This hypothesis breaks down the methodology brown-headed cowbirds use to ensure that their young are raised. This methodology consists of the utilization of violence and terror to kill off other chicks in the nest to instill fear within the mother of the nest. Brown-headed cowbirds are frequently observed bullying warblers by physically removing them from their own nests to lay their eggs and effectively parasitize their nests. They also peck at fathers and mothers attempting to defend their nests and young. In an attempt to combat this parasitism, warblers build nests on top of parasitized nests as to not waste their resources on members of another species. This leads to the presence of "stacked nests." A warbler has also been observed dumping a brown-headed cowbird egg out of her nest, this however was an isolated incident.
Next, I discussed death where we primarily discussed the importance of knowing death rates in terms of evaluating the stability of a population. Understanding population rates and stability is important for numerous reasons; indicating changes in environmental factors (climate change), presence of an invasive species, and changes within a species in terms of breeding. Factors contributing to population rates are mortality rates, sex ratios, breeding behavior, death rates, and emigration rates. For example, a certain species of bird has two different colored feathers throughout two different life stages. Analyzing feather color will lead to a better understanding of at which stage a bird died and why.
In terms of studying sex, we primarily discussed the role of breeding and the process of mate selection. Within a bird species males are typically more brightly colored and bigger in size in comparison to females due to their breeding roles, Males typically have to "show" or offer a display to females to attract them, which means that females select which male they prefer to mate with. During this process, males have to be large because they often have to fight other males off who aim to breed with the same female. When these breeding roles are reversed within species, then the females are generally more colorful and have some physical trait, which enables them to fight off competition. One species we observed has a sharp talon-like spike on her wing, which she uses to ward of competitors.
This tour day in the Burke Museum offered great insight into the behavioral and life cycles of various bird species. Specimens offer great insight into recognizing these patterns and observing how they change over time.

Posted on June 3, 2012 12:44 AM by laurars13