STUDENT INSTRUCTIONS

This data set concerns the possible effects of brood parasitism on the species composition of a songbird community. Your instructor will give you directions about using this file to make figures (graphs) to address this issue.

Background information:

      Some species of cowbirds don't appear to be the best parents. For example, Molothrus badius doesn't make nests and M. ater - the brown-headed cowbird studied here - does not even pair up, defend territory, and take care of its young.

      The Brown-headed cowbird is one of two cowbird species in North America. Both species are brood parasites - they lay their eggs in the nests of other birds. Brown-headed cowbirds are native to grasslands of the Midwest, but clearing of forests has resulted in their expansion across the U.S. The extent of parasitism varies with habitat and with the abundance, breeding behavior, and conservation status of different host species. The negative impact of brown-headed cowbirds on their hosts is very controversial, and there are many conservationists believe that cowbird parasitism is not a major factor in the decline of songbirds in the U.S. (see websites).

      The brown-headed cowbird (Molothrus ater) in North America is a brood parasite with over 200 songbirds. Numerous studies show reproductive losses for individual host species, and therefore it has long been assumed that cowbird parasitism can result in changes in songbird communities. Surprisingly the De Groot and Smith (2001) study is the first to assess whether this brood parasite can change bird community composition. They state their hypothesis as: brown-headed cowbirds change the composition of songbird communities by depressing the number of suitable host individuals.

      This work has management implications in regard to cowbird removal programs designed to protect endangered birds. De Groot and Smith worked in northern Michigan where there is an extensive cowbird removal program to protect the Kirtland's Warbler (Dendroica kirtlandii). This bird only nests in young jack pine forests in northern Michigan. A 1971 census recorded only 201 singing males, and both limited/poor quality of habitat and cowbird parasitism were proposed as responsible for their decline. A cowbird removal program began in 1972. De Groot and Smith compare songbird communities in cowbird removal sites and control areas that are at least 5 km from the removal sites.

      The researchers divided the songbird communities into Suitable Hosts (birds that accept cowbird eggs and feed their young) and Unsuitable hosts (birds that reject cowbird eggs - includes cavity nesters which eat seeds and fruits). They selected 10 Removal sites adjacent to active cowbird traps in operation for 5-11 yrs. Control sites were > 5 km from cowbird traps or any area that had not experienced cowbird removal within the last 5 yr. They matched general habitat characteristics for Removal and Control sites. In 1997 they added Control sites < 10 km from cowbird traps. Site areas were a half circle radius of 1 km.

      Birds were identified by sight or sound on transect lines (four 1 km lines each 60 degree apart). Between dawn and 1000 on all sites birds were counted twice during summer in 1996 and three times in 1997. Cowbirds were measured with additional playback of female chatters to improve likelihood of cowbird detection rates, In addition, nests were sampled (33 in 1996 and 98 in 1997) by visual checks every 3-4 d in 1996 and 3-7 d in 1997. Researchers made sure that nest were checked just prior and after chicks had fledged. Nests that had >1 chick until 1-2 d before fledging were scored as successful.



References:

De Groot, K. L., and J. N. M. Smith. 2001. Community-wide impacts of a generalist brood parasite: the brown-headed cowbird (Molothrus ater). Ecology 82: 868-881