Population dynamics and survival of the Red-necked Grebe Podiceps grisegena: results from a long-term study in eastern Poland

Abstract

Red-necked Grebe (RNG) breeding numbers have been declining in parts of Europe for more than 10 years. We have examined population trends and estimated, for the first time, survival rates of RNGs in a population in eastern Poland in order to explore the mechanisms behind the decline. We counted nesting pairs and recorded breeding success of RNGs nesting on fish ponds. Since 1996 we marked 91 adults and 79 young individually with colour rings and collected resightings of the marked birds. We estimated apparent survival with a Cormack–Jolly–Seber model and used estimates to create a Leslie matrix model for the study population. The population dropped from an estimated 69 breeding pairs in 1995 to a minimum of 12 pairs in 2010, and subsequently remained at 13–16 pairs until 2015. Average annual apparent survival was 0.16 in first-year females and 0.76 in adult females, or 0.18 and 0.79 for sexes pooled, respectively. Site fidelity was high only in adults. Matrix models based on the observed survival rates predicted the observed population decline until 2010. A stable model population was only reached when we increased first-year survival to 0.5. While winter severity had a strong influence on annual survival, low recruitment despite sufficient breeding success was the most important mechanism in this decline. Our observations suggest that natal dispersal, which was not compensated for by immigration, is likely the main proximate mechanism behind the decline.