Veronika Schmidt

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

Pale-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.

Spatial behaviour in the Aquatic Warbler (Acrocephalus paludicola) during mating and breeding@@@Räumliches Verhalten des Seggenrohrsängers (Acrocephalus paludicola) während Partnerfindung und Bebrütung

The range use of both sexes of the Aquatic Warbler was studied during two breeding seasons by radio-tracking. Males used home ranges of up to 8 ha. These home ranges over-lapped by up to 74 %, (mean 51 %) between individuals. During the mating period and prior to nesting, females used isolated sites of 2.8 to 6.4 ha (mean: 4.2 ha) within activity ranges of 100 to 160 ha (mean: 120 ha). Home ranges of different females overlapped by up to 20 %. However, during nest-building and incubation, the average size of home ranges in females was only 1.6 ha. Since no constant ranges were observed, we conclude that Aquatic Warblers are not territorial in the breeding season. In zwei aufeinanderfolgenden Brutzeiten wurde die Raumnutzung von Seggenrohrsängern mittels Telemetrie untersucht. Die ermittelte Größe der Streifgebiete von Männchen betrug in der gesamten Brutzeit bis zu 8 ha. In dem Streifgebiet eines einzelnen Männchens wurden bis zu elf telemetrierte Männchen festgestellt. Die Überlappungen der Streifgebiete telemetrierter Männchen betrugen durchschnittlich 51 %, maximal 74 %. Weibchen nutzten während der Paarungszeit nicht zusammenhängende inselartige Areale von insgesamt 2,8 bis 6,4 ha (Mittelwert 4,2 ha) innerhalb von Aktionsräumen von 110 bis 160 ha (Mittelwert 120 ha). Diese Areale überlappten zu 20 % mit den Aufenthaltsgebieten anderer Weibchen. Während des Brütens sank die Größe der Streifgebiete der Weibchen auf durchschnittlich 1,6 ha. Da im Beobachtungszeitraum keine konstant genutzten Aufenthaltsgebiete vorlagen, schließen wir, daß sich Seggenrohrsänger in der Brutzeit nicht territorial verhalten.