Cowbird parasitism of Pale-headed Brush-finch Atlapetes pallidiceps: implications for conservation and management

Abstract

SummaryPale-headed Brush-finch Atlapetes pallidiceps is a restricted-range species that is threatened with extinction due to habitat loss. The total population of 60-80 individuals achieved a reproductive output of only 0.74 young per breeding pair in 2002. Brood parasitism by Shiny Cowbird Molothrus bonariensis was a major factor reducing breeding success, affecting 38.5% of broods. Parasitism rates reached 50% in an ungrazed reserve, but only 14% on an adjacent grazed plot. The resulting difference in breeding success was not, however, attributable to vegetation parameters used to describe microhabitat use. Cowbird parasitism rates therefore seem to be influenced largely by factors operating at the landscape level. These may include grazing scheme, topography, humidity and host availability. It is suggested that lower species diversity and bird abundance rendered the grazed site less attractive to cowbirds. Current parasitism rates are of great conservation concern due to the low population size of Pale-headed Brush-finch, and the initiation of controlling measures is pressing. Management options described from intensive cowbird control programmes in North America are reviewed and evaluated for their applicability here. To combine the possibility of further data collection with commencement of immediate conservation action, we consider two alternative approaches. Nest monitoring and cowbird egg removal would enable the study of the distribution of parasitism in relation to landscape and vegetation variables, whereas cowbird shooting and nest monitoring might provide a larger short-term benefit to reproductive output. Habitat management, resumption of some grazing in the reserve and cowbird removal should be considered for the intermediate future.

A synopsis of suggested approaches to address potential competitive interactions between Barred Owls (Strix varia) and Spotted Owls (S. occidentalis)

“…Because of the rapid spread of Barred Owls (Kelly et al 2003) (Green and Hirons 1991;D'Antonio et al 2001;Oppel et al 2004), both short-and long-term actions should be initiated simultaneously (i.e., particularly Approaches 2 and 3). It may also be beneficial to initiate other investigations not covered here, such as potential relationships between Barred Owls and other species, or other types of ecological relationships (Korpimaki and Norrdahl 1998;D'Antonio et al 2001;Ekerholm et al 2004).…”

Northern Hawk Owls and Recent Burns: Does Burn Age Matter?

“…Since we used distribution data to define suitable habitat, it has to be cautioned that there is no guarantee that presence of individuals in a certain habitat is related to habitat quality (van Horne 1983, Pulliam 1988, Jones 2001. Cowbird parasitism or interspecific competition by other brush-finch species might reduce the availability of suitable habitat (although the sympatric Stripe-headed Brush-Finch [Buarremon torquatus] uses a different microhabitat, Oppel et al 2004b).…”

How Much Suitable Habitat Is Left for the Last Known Population of the Pale-Headed Brush-Finch?

“…It inhabits heterogeneous semiopen scrubland, where patches of scrub with medium height are interspersed with small grassy clearings. Dense continuous scrub adjacent to open areas is present in most territories, and is considered important for nest sites (Oppel et al 2004b). Territories were established in patches of pure deciduous or bamboo scrub.…”