Prenuptiaal gedrag, voedsel en rui van overzomerende Kraanvogels Grus grus in West-Drenthe in 1999

After several years of increasing frequency of summer observations, three Common Cranes settled in western Drenthe in 1999; an adult pair and a -presumably 3rd calender-year- male (the latter sex based on size comparison with the adult pair). The area consists of protected heathland (Wapserveld), a brook valley (Vledder Aa, width 400-500 m, partly nature reserve with extensively used grasslands) and farmland (mainly grassland). The adult pair stayed from 20 May through 13 June, roosted on the heath and foraged on the heath and in farmland in the vicinity (up to at least 12 km). Before sunrise and after sunset, the pair was seen displaying (but no copulations witnessed) and heard unison calling. After 13 June, the pair probably disappeared to the large peat-moor Fochteloerveen, some 17 km away, where they formed a group with another pair; the latter had been present at Fochteloerveen since April, also showing prenuptial breeding behaviour (including copulations). The immature male stayed from 20 May through 12 October. During this period, it symmetrically moulted a series of primaries (at least 10, both innermost and outermost) and secondaries (>20) and numerous mantle feathers (Figure 2). It refrained from flying from late June through mid-August, foraged in a restricted part of the brook valley (covering 32 ha in 146 days) and roosted on the nearby heath. The distance between roost and foraging site of 900-1000 m was cautiously covered on foot, well before sunrise and well after sunset. From mid-August onwards, part of this trip was made by flying. It is thought that energetic costs of moult were met by refraining from flying, decreasing activity during foraging (very restricted feeding range) and choosing a feeding area where disturbance from people and predators was least. High ambient temperatures in summer 1999 may also have reduced thermostatic costs of moult. In May and early June, the bird mainly foraged in wet parts of the heathland, but after mid-June (probably coinciding with start of moult), it changed towards foraging in rough grasslands along the Vledder Aa. During a single day of foraging (from before sunrise till after sunset; Figure 1), the bird covered only very small distances, during which it was grazing continuously apart from short spells (10-40 minutes) of resting. These resting bouts were thought to coincide with digestive bottlenecks. Dropping analyses confirmed that the bird mainly fed on leaves of Lolium perenne and leaves and roots of Rumex acetose. In the course of the summer, the frequency with which seeds of Rumex acetose and Plantago major occurred in the droppings increased. Insects were infrequently found in the droppings, mainly beetles and grasshoppers (Table 1). Vertebrate prey was rarely taken; frogs, voles and 5-day old nestlings of a Skylark were either seen captured or found in droppings. The future of Common Cranes in this part of The Netherlands depends on protected areas with undisturbed breeding and feeding areas of sufficient size. The present policy to allow and even promote recreation in hitherto quiet nature reserves is a serious threat to the presence and breeding success of birds like the Common Crane.