LEYS HN & WILDE JJFE DE (1971) The occurrence of the Great Crested Grebe Podiceps cristatus in the Netherlands. LIMOSA 44 (3): 133-183.

In this article the data collected by about 400 observers, throughout The Netherlands have been used. The breeding locations of the Great Crested Grebe Podiceps cristatus L. are distributed over large parts of the Old World, Africa, Australia and New Zealand (Fig. 1). Fig. 2 shows the breeding density (pairs per 1000 km2) in part of Europe. The Dutch water and marsh areas can be divided into several areas important for Podiceps cristatus. They are alle more or less important as breeding areas of Podiceps cristatus. In Fig. 3 the area of open water as percentage of the total area of land and water is illustrated in the different habitats. Probably Podiceps cristatus was unknown in Holland from 1500 until about 1600. After 1700 until about 1900 the Great Crested Grebe seems to have become more common. However, large numbers of this species were killed at that time, because the feathered skin of the breast and belly was used as fur and mainly exported to Great Britain. Also fishermen levied a heavy toll on these birds. Probably after 1925 the breeding population of Podiceps cristatus increased. In 1932 Thysse (1932) estimated the number of breeding pairs to be about 300 or less (see also Table 2 and 3). The counting in 1966 gives. for the whole Netherlands a total of 2980-3200, and for 1967 3300-3400 breeding pairs (Table I). For several reasons these numbers are under estimated. Counts of swimming Great Crested Grebe from the shore or from a sailing or rowing boat in the breeding season, when there are not yet many juveniles, probably gives numbers about half of the actual number. Only counting from the shoreline on small isolated ponds, narrow canals and on small waters in reclaimed low peat areas gives a fairly accurate result. The most exact data of the total number of breeding pairs can be obtained during the breeding season, when there are many juveniles (Table 2). From comparison of accurate counts with previous estimations in the same area it can be concluded that the actual number in 1966 was 8-10% higher than the estimations. In 1967 an increase of 6%-10% was necessary in most areas, while an increase of about 20% had to be made in a few remaining areas. Probably the exact number of breeding pairs numbers 3300-3500 in 1966 and 3600-3700 in 1967. The density of breeding pairs is not always the same per area of open water (Fig. 5 and Table 3). The relative number of breeding pairs increases in proportion to suitable small isolated habitats which approach about 10 ha or less (Fig. 6). There is a correlation between the surrounding vegetation' of the nest sites and the nesting material (757 nests). Nest sites were found in 22 different types of vegetation. from which Phragmites communis, Scirpus lacustris and Typha angustifolia numbered about 70% (Table 4). Also important were nests between willow coppices of which the branches were hanging in and over the water (13 %). Extremely rare were nests on dry land, only 13 have been found so far. Building material of the nests consists of many different plant species, but also paper, shells, plastics and so on. The main building materials in 757 nests were Phragmites (83%), Typha angustifolia (29%), Scirpus lacustris (23%), roots from these plants etc. (11 Yz %), rotten waterplants (11 %), roots from Salix and Alnus (6%), branches and twigs (4Yz %), Nymphaea and Nuphar (3 %). At least 40 plant species and about ten kinds of other material were observed. In the Netherlands the average size of about 700 clutches observed in 1966-1969 was 3.45 per nest. The average size of 592 complete clutches was 3.50 per nest from May until July. After July the average complete clutch decreased to about one half of the value before that month. The average clutch size differs slightly in some areas from the country average (Table 5). The percentage frequency distribution of 592 clutches in the Netherlands is compared with data from Estonia, Belgium, Bohemia, Switzerland and Great Britain (Table 5). We found that complete clutches of 1 and 2 eggs were laid mostly at the end of the breeding s,eason, probably by birds which had already started breeding in their first year after hatching. Clutches of 7 eggs and more (up to 9) often come from two ~ ~ (G1utz von Blotzheim, 1966), but we found that it was also possible for one female to lay a clutch of 7-9 eggs. Measurements of 273 eggs were made in 6 areas from April through May and sometimes in June. In alleareas the earlier laid eggs (May) were a little larger than the eggs laid in June and probably in July and August (Table 6). The measurements of the eggs of the Dutch Great Crested Grebes are about the same as the eggs from Belgium and Great-Britain (Table 7). The main breeding season in the Netherlands is mainly in May and June, though nests were also found in the beginning of March and April and also in September and very rarely in October. Breeding success is rather low, though breeding pairs with 5 full grown juveniLes do occur, especially on isolated ponds, lakes etc. with only one pair. Counts in 54 areas (except areas with only one, two or three pairs) in July and August 1967 resulted in 1762 adults and 1600 juveniles, thus an average of 1.96 juveniles per breeding pair (maximum 3.92, minimum 0.41). Great Crested Gr'ebes breeding in colonies mostly have a very low reproduction rate. In those cases we observed 0.41-1.20 full grown juveniles per breeding pair. Large groups (concentrations) of Great Crested Grebes can be observed in all seasons especially in July and August and up till November. Such concentrations often occur on several places on the lake IJsselmeer and number up to 2000-3000 individuals. In spring, smaller groups are observed in the neighbourhood of the breeding areas. These numbers are closely correlated with the actual breeding population of the surrounding habitats. Ringing results from the Netherlands are very scanty (only 10 recoveries). However from the combined results from other European countries, it is apparent that Dutch birds, and also birds from Denmark, Sweden, Norway and probably Finland, migrate southwards. The Dutch population winters partly along the French and British coasts, partly on large lakes free from ice, in East France and Switzerland and perhaps in the Mediteranian (Fig. 8). In the Netherlands during the winter, we still find a few Podiceps cristatus on waters which are not frozen. In the breeding season we find a relatively high density of the breeding populations in suitable habitats. This density is the highest of all West European countries. Increase in water recreation will disturb suitable breeding places. In any case the Grebes will be confined to relatively quiet areas, the process resulting in the future in a probable decrease of the number of breeding pairs. It is necessary to close a number of places in open water recreation areas to speedboats. If this is not done, the Great Crested Grebe wil disappear completely from such areas. Great Crested Grebe Podiceps cristatus

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