Raouf and Colleagues (2006) Found That Cliff Swallow Colony Size

Raouf and colleagues (2006) found that cliff swallow colony size is correlated with levels of glucocorticoid hormones. They further demonstrated that the stress being displayed was related to the presence of parasitic swallow bugs rather than a response to other aspects of group living, such as competition for food. They accomplished this by

A) capturing each bird individually in some colonies and gently removing all swallow bugs by hand.
B) fumigating some colonies in order to kill the parasites.
C) reducing the size of some colony sites to encourage instances of allopreening among group members.
D) supplementing the normal foraging efforts of some colonies by providing additional food.