Survival, reproduction, and immigration explain the dynamics of a local Red-backed Shrike population in the Netherlands

Abstract

Populations of many bird species strongly declined in Western Europe in the late twentieth century. One such species is the Red-backed Shrike in the Netherlands. In one of the last strongholds of this species, the Bargerveen Reserve, the breeding population flourished in the 1990s due to rewetting management. However, further development of the area has led to a decline in breeding numbers such that the population is now less than half the size it was in the 1990s. Here, we analyze the vital rates of the Red-backed Shrike population in the Bargerveen. In 2001–2008, nestlings in this population were individually marked, and resighting data was collected during the breeding seasons of 2002–2009. We used estimates of vital rates based on monitoring in 2001 until 2009 to diagnose the population dynamics. Mark–recapture data were analyzed with the program MARK. The most parsimonious model gave age- and gender-specific survival probabilities of 0.12 for first-year females, 0.64 for older females, 0.20 for first-year males, and 0.54 for older males (with overlapping confidence intervals for the gender-specific adult survival values). The estimated yearly resighting probabilities were gender specific, with a higher probability observed for males (0.81) than for females (0.53). For 2001–2009, we computed an average number of offspring per breeding pair of 2.91 (with 72 % of the pairs breeding successfully). Using these vital rates, we parameterized a simple matrix model. The resulting yearly growth was 0.80. Adult survival had an elasticity of 0.83, while juvenile survival and reproduction both had an elasticity of 0.20. Because the population numbers have stabilized since 2005, the observed yearly population growth suggests that 20 % of the Red-backed Shrikes breeding in the Bargerveen are immigrants. Comparison with data on other Red-backed Shrike populations indicates that juvenile and adult survival rates can be improved in the Bargerveen Reserve. Appropriate management measures to accomplish this are discussed.