The current distribution of the Northern Wheatear in the Netherlands is largely confined to coastal dune grasslands. This habitat is threatened by loss of natural dynamics, excessive atmospheric nitrogen deposition and declining Rabbit Oryctolagus cuniculus populations. We studied the effects of these pressures on the population numbers, breeding success and survival of Northern Wheatears in one of its last strongholds, the Noordduinen, in the period 2007- 19.

The number of territories fluctuated between 46 and 67, increasing between 2007-12 but decreasing afterwards (Fig. 2). The recent decrease is particularly apparent when excluding territories in which we found no nest. The proportion of solitary males increased from 1 in 2014 to 16 in 2018. In 2019 the population slightly recovered. The number of Rabbits peaked in 2006, but crashed afterwards, with particularly low numbers in 2015-18 (Fig. 1). Grass encroachment progressively occurred from 2012 onwards. Based on multistate survival modelling, using all resightings of 214 colour-ringed adults and 1519 nestlings, we calculated an average annual survival of 57% for adult males, 51% for adult females and 24% for juveniles (Appendix 1). Resighting probabilities are high, on average 93% for males and 90% for females. The best model indicates different survival and resighting rates for males, females and juveniles, and between years (Appendix 1). Fig. 4 shows the annual survival rates of an alternative model, in which the differences between age and sex classes are not constant over time, but vary between years. We found large differences in survival between years, but no evidence for a trend during the study period. Average laying date of first clutches is 4 May, with annual averages ranging between 1 – 12 May (Appendix 1). We found no evidence for a trend in laying date during the study period. Most first clutches contain five (N=135) or six (N=175) eggs, rarely seven (N=31) or eight (N=1). Average clutch size differs between 4.9 and 5.9 annually (Appendix 1). Early clutches are on average larger than late clutches. Number of fledged young per successful nest is 4.7 for first broods and 3.7 for second and replacement broods. The number of fledged young per territory was on average higher in 2007-11 (3.9), before the onset of the decline in numbers, than in 2012-18 (2.6) (Fig. 3). While the number of fledged young per successful nest was stable during the entire study period (4.6 in 2007-11 versus 4.8 in 2012-18), the proportion of successful territories strongly declined (Appendix 1). In 2019 breeding success recovered (Fig. 3). Nest predation became apparent from 2012 onwards, initially by Red Fox Vulpes vulpes (up to at least 15 clutches in 2015), from 2015 onwards by a mustelid, probably Polecat Mustela putorius (up to 9 clutches and 5 breeding females in 2017). After the start of active nest protection in 2015 nest predation by foxes gradually disappeared. Our findings suggest that a decrease of suitable foraging habitat due to grass encroachment in response to declining Rabbit numbers, in combination with increased nest predation rates, are the main drivers of the local population decline of Northern Wheatears after 2012. In a situation with low Rabbit densities, active nest protection and habitat restoration measures are needed to counteract these adverse effects.