Blackbird dominance and habitat loss

Abstract

Yellow-headed (Xanthocephalus xanthocephalus) and red-winged (Agelaius phoeniceus) blackbirds coexist in marshes across North America. Pictured here at Iona Beach Regional Park in Vancouver, British Columbia, males of each species compete for nesting areas. In wetlands across their overlapping ranges, yellow-heads are dominant over red-wings, pushing them out of valuable marsh real-estate to secure the best nesting places.

Unfortunately, since the construction of Vancouver International Airport in 1968 and its expansion in subsequent years, both species have lost substantial extents of marsh habitat. Given their dominant–subordinate relationship, one might expect that habitat loss should disproportionately impact the subordinate red-wings, since they get last choice at nesting habitat, which might limit their reproductive success. Today, however, Iona Beach supports a healthy and abundant population of red-wings, and usually hosts only a few yellow-heads. Indeed, over the past half-century, the population of yellow-heads at Iona Beach has decreased ~95%, down from an estimated 70 individuals in 1970 to only 1–3 nowadays (Campbell RW, Dawe NK, McTaggart-Cowan I, et al. 2001. The Birds of British Columbia. Volume 4. Royal British Columbia Museum Victoria. Vancouver, Canada: UBC Press).

Despite their dominance, the yellow-headed blackbirds of Vancouver have seemingly been more negatively impacted by habitat loss and degradation than their subordinate red-winged cousins. Does their dominant behavior make yellow-heads more vulnerable to change? Are red-wings more tolerant of anthropogenic disturbances? Over the next decade, the potential extirpation of yellow-headed blackbirds from Iona Beach calls into question which blackbird genuinely is the dominant species. Of course, none of this stops the remaining few yellow-heads from bullying the abundant red-wings out of prime nesting space. At least population decline has not adversely impacted their yellow-headed ego!