OOSTERVELD E (1999) Reproductive succes and immigration of Whinchat Saxicola rubetra in Geelbroek (Drenthe): keys to increase and decrease?. LIMOSA 72 (4): 143-150.

In the past decades, numbers of breeding Whinchat in The Netherlands have been subject to a major decline, probably mainly due to losses of suitable breeding habitat. Although, like in other passerine species, numbers are known to fluctuate according to precipitation in their main wintering areas in the Sahel in Africa (winter survival increases with rainfall), the recent decline in the 1990s cannot be explained with this factor. Additional causes might be found with population biological parameters, but little is known about which key-factors dominate local populations in the breeding range. This paper deals with the population dynamics of a Whinchat population in the nature reserve Geelbroek, near Assen in the province of Drenthe. Data were collected during an annual breeding bird survey and analysed with respect to the number of territories, number of (successful) breeding pairs, number of young fledged and the occurrence of unpaired males.
      The breeding population in the study area ranged from 3-15 territories. From 1989 onwards, there was a steady decrease (Fig. I), in line with the national figures in similar habitats. Especially in the 1990s these have shown an ongoing decline. In some years up to 45% of all territories at Geelbroek consisted of solitary males (Tab. I), which generally arrived later in spring than the males of a breeding pair (Tab. 2). Reproductive success, expressed as the number of young fledged from all eggs laid, was estimated to range from 36 to 54% on average, with succesful pairs having on average 2.9 young (Tab. I). Both the occurrence of solitary males and the breeding results showed large annual fluctuations and did not correlate with year-to-year changes in the size of the local breeding population. As the local reproductive output and habitat characteristics in the study area do not seem to be limiting factors (the habitat for Whinchat in the reserve even improved), it is discussed if variation in breeding numbers might be the result of immigration. It is known that Whinchat populations consist for a large extent of first-year breeders. Since first-year breeder show a low return rate to their native breeding sites, any local population depends on immigration. Furthermore, the late arrival of adult males which remain unpaired, supports the idea of an extra influx from elsewehere in some years. As breeding populations in the Dutch core areas as well as those in Germany and Sweden have decreased in the 1980s and 1990s, a lower immigration rate from thes~ areas might be a key to explain the negative population developments observed locally.

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