Aantalsverloop en broedsucces van de Dodaars Tachybaptus ruficollis

Standardised breeding bird censuses in western Drenthe and Dwingelderveld (24,170 ha) in 1967-94 revealed marked population fluctuations in the Little Grebe, especially since the early 1980s (Fig. 1). The overall increase since then could be partly attributed to habitat management in nature reserves, such as raising the water level, in combination with a series of mild winters. Cold and severe winters resulted in short-term declines, particularly after the winters of 1978/79 and 1984/85-1986/87. Population size correlated negatively with the frost index of Unsen (based on number of days in which minimum temperature remained below 0°C, ditto in which maximum temperature remained below 0°C and ditto in which minimum temperature remained below -10°C, during November through March). No correlation was found between rainfall (in mm, during Nov-Mar) and number of breeding pairs in the following breeding season. Breeding performance was studied in 1995. Of 35 nests found, 32 were situated on fens and 3 on flooded grassland with Juncus effusus. Breeding pairs showed a preference for breeding in fields of Carex rostrata or beneath overhanging branches. All in all, 24 first layings, 7 second layings and 4 repeat layings were found, with onset of laying covering the timespan between late April and late July (Fig. 2). Clutch and brood size were largest in first layings, smaller in repeat layings and still smaller in second layings (Table 1). Occasionally, a third laying may be produced, as was once found in 1994. Brood size (young >4 weeks old) equalled clutch size in only seven out of 24 successful nests. The remaining pairs lost 1 (12x), 2 (lx) or 3 (4x) young within four weeks after hatching. Of 74 hatchlings, 48 young were still alive at the age of 28 days (65%). Most young disappeared during their first week of life, probably through emaciation. Breeding success was high, with 16 of 24 first layings being successful (raising 58 young), 4 out of 7 second layings (raising 11 young) and 3 out of 4 repeat layings (raising 7 young). The chances of a nest to survive till hatching was estimated at 56% (using the Mayfield-method, but notice small sample size). Nest failures could be attributed to nest predation (5x. but probably 8x), unstable nest construction (lx), heavy rainfall (lx) and drought (lx). In three other nests, a single egg did not hatch, whereas one egg disappeared during brooding in another nest (remaining three eggs hatched). Population size seems to depend on weather conditions in winter, the species being particularly susceptible to hard winters. The rate of increase or recovery after severe winters depends on availability and suitability of habitats for breeding (can be improved by management, but also relies on rainfall), reproductive rate in preceding breeding seasons and survival rates in winter.