OVERVIEW OF THE ECOLOGICAL BACKGROUND

      The brown-headed cowbird (Molothrus ater) in North America is an obligate or generalist brood parasite of over 200 songbirds. Numerous studies show reproductive losses for individual host species (e.g. Strausberger and Ashley 1997). It has long been assumed that cowbird parasitism can result in changes in songbird communities, but surprisingly the De Groot and Smith (2001) study is the first to assess whether this brood parasite can change overall 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. In 1971, a 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. In this study, De Groot and Smith compare songbird communities in cowbird removal sites and control areas.

      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). 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 assessed 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.

      De Groot and Smith did not find significant differences in bird community composition in controls and areas where cowbirds have been removed for 5-11 yrs. They propose that cowbirds affect bird communities in more isolated places with limited immigration. They conclude that: "If brown-headed cowbirds influence songbird community composition only to a moderate degree when they are dominant members of the songbird community, they are not remarkable in their ability to influence community composition. Our results provide little support for adding cowbirds to the short list of species (Hurlburt 1997) that regulate community structure disproportionately in relation to their abundance. Thus, while cowbird removal has benefited some endangered species (Griffiths and Griffiths 2000) there is little reason to believe that it will improve the general health of songbirds". (see article for references)


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

Strausberger, B. M. and M. V. Ashley. 1997. Community-wide patterns of parasitism of a host "generalist" brood-parasitic cowbird. Oecologia 112: 254-262.