Water temperature and phytophenology indicate the earlier onset of oviposition in Eurasian Dipper (Cinclus cinclus) from the Swiss Lowlands

Initiation of reproduction was investigated in Eurasian Dipper (Cinclus cin clus) along riparian habitat of tributaries flowing into lower Lake Zurich (Switzerland). Total length of these drainages is 40 km and Dippers nested from 410 - 680 in asl. From 1992 to 2000 about 46 pairs per year had 552 su ccessful broods with nestlings of at least 8 to 12 days of age. Meterologic al data (i. e., air temperature, days with rain, amount of rainfall, amount of sunshine) and water temperature in a study area river were obtained fro m government weather stations. Phytophenological data, i.e. full flowering of Coltsfoot (Tussilago farfara), Wood Anemone (Anemone nemorosa), and Dand elion (Taraxacum officinale), were also available from stations within the study area. Dippers initiated breeding in February or March of each year. w ith considerable variability among years. Earliest oviposition was 9 Februa ry 1995, and again 1997, while latest was 9 March 1992. The median date eac h year for appearance of the first 25 % of all broods was significantly cor related with February water temperatures (Spearman rank correlation. p < 0. 001), with February air temperatures (p < 0.002), and with the flowering of two early phanerogams (Coltsfoot and Wood Anemone; p < 0.025), but not Dan delion (p < 0.1). According to historical data, the initiation of reproduct ion for Dipper at Lake Zurich occurs now earlier in the year than it did in Britain, Germany, and Switzerland. This may be due either to global warmin g, or (more significantly) to an increase in water temperatures as a result of urbanisation. Warmer waters accelerate life cycles of aquatic arthropod s and generate earlier emergences of insects that form the prey base of the Dipper. Since avian reproduction is cued by trophic resources, an earlier appearance of aquatic insects will concomitantly induce an earlier breeding season for the Dipper.