The Importance of Reedbeds and Riparian Areas for Cetti’s Warbler Cettia cetti throughout its Annual Cycle

Abstract

This study describes the importance of reedbed and riparian habitats for the annual cycle of Cetti’s warbler Cettia cetti. We examined the seasonal variation in population structure and body condition using a long-term data set (11 years) from bird ringing. Because seasonal variations in the differential use of each wetland type should be influenced by food resources and shelter conditions for roosting, a short term study (1 year) was performed to assess the diet, trophic niche and health condition of Cetti’s warbler, and to examine differences in the abundance of food resources and temperature between the two habitats. Results revealed that reedbeds are very important for Cetti’s warbler, supporting large numbers of (mainly) juvenile females during the late summer and autumn periods. Fecal analysis showed that during late summer predator insects (mostly Coleoptera) were more important for Cetti’s warblers on the reedbed, while Phytophagous insects (Hemiptera) were more important for Cetti’s warblers on the riparian habitat. Stable isotope mixing models from blood and feathers samples corroborated these results and revealed the opportunistic foraging behaviour of Cetti’s warbler. The variation on the δ13C values of juvenile secondary feathers was higher for the reedbed, indicating a wider provenance for birds that move into the reedbed in late summer/early autumn. The δ13C values for the riparian area had a smaller variation suggesting that these birds were less dispersive. Overall, the higher abundance of higher trophic level food resources, and higher minimum temperatures for roosting on the reedbed, makes this habitat particularly attractive for Cetti’s warbler after the breeding season.