Daniel R. Petit

Impact of Pasture Development on Winter Bird Communities in Belize, Central America

We studied bird use of actively grazed and abandoned (2-4 years) pastures during February 1990 in Belize, where pastures often are maintained by repeated mowing. A total of 46 species was observed in the two pasture types, with 15 species detected in grazed and 39 in abandoned. Species richness for both migrants and residents was lower by >50% in grazed pastures. Overall abundance of birds was lower by nearly 70% in grazed pastures compared with the more structurally diverse abandoned pastures (P < 0.001). Based on data from a concurrent study on bird use of forest interior habitat in Belize, 2-4 times more resident species would be expected in nearby broadleaved forests compared to an equal census effort in grazed pastures. In contrast, only about 1.5 times more migrant bird species would be predicted in broadleaved forest interior than in grazed pastures. Foraging guilds were not equally represented between pasture treatments; most notable was an absence of frugivores and nectarivores in grazed pastures maintained by mowing. Our results indicate that retention of some shrubs and overstory trees may reduce the negative effects of pasture development of winter bird populations in Central America.

Fecal sac dispersal by Prothonotary Warblers: Weatherhead's hypothesis re-evaluated

Data on removal of fecal sacs by Prothonotary Warblers (Protonotaria citrea) were used to test Weatherheads (1984) hypothesis that birds carrying fecal sacs away from the nest will vary their departure directions more widely than will birds not carrying fecal sacs in order to prevent attraction of predators. We recorded the departure direction of adults and whether or not they carried fecal sacs, as well as the distance flown to drop a fecal sac. Fecal sacs dropped over land were not carried significantly farther than those dropped over water. Although the distribution of directions flown with fecal sacs was significantly different than that of directions flown without fecal sacs, Prothonotary Warblers did not vary their departure direction more when carrying fecal sacs than when not carrying sacs. In fact, the distribution of trips without fecal sacs was more varied than trips with fecal sacs. Our results contradict the results of Weatherhead (1984), possibly because of differences in data analysis, and do not support his hypothesis of fecal sac dispersal.