FACULTY NOTES

The figures show that the effect of birds is proportional to the flock size. Note that the trial with the large cuckoo appears as an outlier in the top figure because a large bird has a larger effect (eats more) than a small bird. When flock size is measured as biomass (lower graph), this trial fits nicely in the overall trend.

Some students may have difficulty in understanding the relationship between the data in the two parts of Figure 2. The instructor may need to assist some groups by clarifying the concept of biomass and helping them understand why one large bird (that apparently eats a lot) would appear as an outlier when the number of birds is used as the independent variable and how using flock biomass corrects this.

According to Figure 2, birds often reduce army ant foraging success by about 1-3 insects/10 minutes, or about 75-200 insects in a 12-hour day. Using the average mass of 0.035 g/insect, this constitutes a loss of 2.6-7 g of food, or a 5.6-15% reduction in food intake.

The estimates of the significance of antbird parasitism on the army ant colony will likely vary from group to group. Students may come up with a variety of ways to do the calculation. For example, some may convert the daily intake by the ants into the number of large insects and compare that to what birds take. Answers will also vary depending on how "typical" flock size is estimated.

Army ant colonies are probably energy limited and live close to the break-even point. Ant-following birds can decrease daily foraging success by up to 15%, which is probably ecologically significant. This loss may slow colony growth rate, increasing the time required to reach the minimum size for fission (colony reproduction). This in turn means that the queen would have a greater chance of dying, increasing the probability of colony extinction. In addition, the loss of primarily litter arthropods to birds could shift army ant foraging more towards hymenopteran brood, causing changes in the forest community composition (Wrege et al. 2005).

A possible essay question for assessment:

Consider the broader implications of this experiment. What consequences might the bird-ant interaction have for an army ant colony and the forest community in which it lives?

Upper-level students should be able to come up with some ideas to answer this question. For less experienced students, this question could instead form the basis for a class discussion, guided by the instructor.