Diet of the Cantabrian Capercaillie: Geographic Variation and Energetic Content

Ardeola 47(1), 2000, 77-83

DIET OF THE CANTABRIAN CAPERCAILLIE: GEOGRAPHIC VARIATION AND ENERGETIC CONTENT

Ana Esther RODRÍGUEZ* & José Ramón OBESO*

SUMMARY.—Diet of the Cantabrian Capercaillie: geographic variation and energetic content. The diet of the Cantabrian Capercaillie Tetrao urogallus cantabricus was examined by fecal analysis. Droppings were sampled from April 1998 to May 1999 around 23 leks that were grouped in five zones. Winter diet consisted of beech buds, pine needles, buds and catkins of birch and holly leaves. In each zone, the birds fed almost ex- clusively on one of these species, so winter diet depended on local availability. During spring the consumption of beech buds increased in all sites, and later in the season a high diversity of herbaceous plants was in- cluded in the diet. The most important food during summer and autumn was bilberry. Considering the resources that were used as monodiet, beech buds showed both the lowest caloric content and highest the fiber content. The energetic problems derived from winter and spring consumption of beech buds are dis- cussed, and we propose that under certain ecological condition this monodiet might be a limiting factor. Key words: Cantabrian range, diet, digestibility, Tetrao urogallus cantabricus, Tetraonidae.

RESUMEN.—Dieta del Urogallo Común cantábrico: composición, variabilidad espacio-temporal y ren- dimiento energético. La dieta del Urogallo Común cantábrico Tetrao urogallus cantabricus fue estudiada me- diante el análisis de heces recolectadas entre abril de 1998 y mayo de 1999 en las proximidades de 23 canta- deros que agrupamos en cinco zonas. La dieta invernal consistió en el uso de diferentes especies arbóreas: brotes de haya, brotes y amentos de abedul, acículas de pino y hojas de acebo fueron los recursos más utilizados. En cada zona los Urogallos Comunes utilizaron de forma casi exclusiva una de estas especies, de ma- nera que la dieta dependió de la disponibilidad local. Entre la vegetación del suelo destacan la brecina y al- gunos helechos. La dieta primaveral se caracterizó por un aumento generalizado del uso de brotes de haya y posteriormente por la inclusión de diversas especies herbáceas. El arándano fue el alimento principal duran- te el verano y el otoño. De todos los recursos utilizados en monodieta, los brotes de haya fueron los de menor contenido calórico y los de mayor contenido en fibra. Se discuten las repercusiones energéticas que tiene el consumo casi exclusivo de brotes de haya en invierno y primavera, indicando que la dieta podría ser un factor limitante bajo determinadas condiciones ambientales. Palabras clave: cordillera Cantábrica, dieta, digestibilidad, Tetrao urogallus cantabricus, Tetraonidae.

INTRODUCTION dators, which might increase in fragmented ha- bitats (Kurki et al., 1997); competition with Diet of birds determines not only resource ungulates (Klaus & Bergmann, 1994; Moss & acquisition, but also habitat selection (Wiens, Picozzi, 1994; Picozzi et al., 1996) and climatic 1989). The diet of threatened species has parti- change (Moss, 1985; Moss & Picozzi, 1994). cular interest because it might be a limiting fac- The effects of these factors are likely to vary tor and, at the same time, supplies information according to geographical location, and other about habitat selection, which is essential to an possibilities, such as diet as a limiting factor, appropriate management of the species. should also be considered. Capercaillie populations have been declining The Cantabrian Capercaillie T. u. cantabri- and their ranges have been contracting during cus has been declining during the last two de- recent decades but the causes are still unknown cades and, although there are few data on Can- (Storch, 1997). The most frequently reported tabrian Capercaillie population trends in the limiting factors for capercaillie are: habitat re- literature, Purroy (1997, 1999) estimated de- du