Official journal of the Netherlands Ornithologists' Union
|Swennen C. (1976) Populatie-structuur en voedsel van de Eidereend (Somateria mollissima) in de Nederlandse Waddenzee. ARDEA 64 (3-4): 311-371|
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The establishment of the Common Eider as a breeding bird in the Wadden Sea area is of fairly recent date. Starting with the first settlement on Sylt between 1785 and 1805 there are four periods during which successful colonisation occurred (Table 1, Fig. 1). At first new colonies turned up on the North-Frisian islands, then, skipping the East-Frisian islands, Eiders also settled on the Dutch or West-Frisian islands (Joensen 1973, Bauer & Glutz 1969, Wilcke 1956). The size of the Dutch colonies increased quickly. In 1930 there were 30 nests on the island of Vlieland; in 1940 they had increased to 350. After 1940 new colonies also appeared on the other Dutch islands and in 1950 the total number of nests was appr. 1,140. In 1960 there were as many as 5,750 nests, but then a severe decline set in (Swennen 1972) caused by a huge mortality among breeding ducks due to poisoning by pesticides, taken up with food before the breeding season started. The poison had been discharged into the river Rhine and subsequently transported northwards by the residual currents along the Dutch coast into the Wadden Sea. Mortality was especially heavy in the western part of the Wadden Sea nearest to the Rhine outflow and tended to decrease to the east (Fig. 2). Discharge of the pesticides concerned was discontinued in 1965 and after 1968 no cases of this particular poisoning have been observed. In 1968 when the effect of the poisoning was at its maximum only 1,329 nests were left but after that year a gradual recovery had been recorded (Table 2). Figure 3 represents the course of events in the two main colonies, those on the islands of Vlieland and Terschelling. The more recent Terschelling colony in particular demonstrates clearly the above cited events, namely a period of exponential increase, a strong decrease due to poisoning during the sixties, followed by a period of renewed increase. The Vlieland colony, the elder of the two, had a starting phase with less than 10 nests in the period between 1906 and 1928. After the breeding area had been given the status of a nature reserve, including guarding during the breeding season, a phase of exponential increase set in, lasting till 1937, when the numbers became more or less stationary till about 1944. Since that year the development was more or less similar to that on Terschelling. Development in the North Frisian colonies included a good many ups and downs. At the end of the sixties some 860 nests were counted, divided over several colonies. This remarkably slow increase as compared to the Dutch colonies is probably due to shooting in the Danish waters. Every winter the number of Eiders killed in Denmark is at least 7 times that of the local breeding populations and includes a high proportion of winter visitors (Paludan 1962, Joensen 1974). In the Netherlands the Eider is protected all the year round. On the Dutch Frisian Islands many nests are built under or near shrubs with a height of +.to 1+, metres. They are seldom found under higher deciduous trees present on the islands. However, on the island of Vlieland many Eiders nest till in the centre of both low and higher coniferous plantations. Moreover, nests are built in heather, marram, reed-beds and, to a smaller extent, on salt marshes and on the debris washed ashore by winter floods. Females tend to nest in subsequent seasons not only on the same island, but also more or less in the same place. In over 97% of the cases studied the nesting site of a female was found less than 100 m from the spot in the preceding year (Table 3). Contrary to the statement of Bauer & Glutz (1969), a high percentage of nests were even found in the same hollow in subsequent seasons. A study with experimental hollows, provided with fragments of egg-shell, or empty, indicated that the Common Eider shows a strong tendency to nest in an existing hollow. Whether these hollows were natural and contained old nests or were experimental ones, and whether they contain egg-shell fragments proved to be unmaterial (Table 4). Though the ducks tend to return to the same part of the colony and use old nests, this does not mean that they always use their 'own' nest. In the Vlieland colony the flea Ceratophyllus garei is found in practically all nests. After hatching hundreds of larvae crawl about in the litter, which pupate in the bottom of the nest and spend the winter there, as shown by sampling. There are no indications that the Eider is much+ troubled by these ectoparasites or that it influences breeding results. The fleas are scarce or absent in the nests in dry marram dunes. Ringing results point to a complicated composition of the Eider population in the Dutch Wadden Sea. Adult ducks, adult drakes and immature birds all show different distributional patterns through the year. The ducks present during the breeding time in adult plumage breed, moult and winter in the Wadden Sea area, and may be considered as permanent residents. (Figs. 5 and 6). Most drakes present during the breeding season in adult plumage (criteria: no juvenile feathers left, complete white wedge on the head), of which it may be supposed that they have mated with the breeding ducks, are recovered in later years in the same colony during the breeding season. Some drakes, however, join other colonies as shown by ring recoveries (Fig. 9). Recoveries from Sweden and Finland date from the breeding season and those from Danish waters mainly from the winter months. For all seasons, however, the majority of the recoveries come from the Dutch Wadden Sea area. Therefore, the ringing results indicate a considerable bond with the place of ringing and the majority of the drakes to behave as residents. However, a certain degree of 'abmigration' does exist, after which the bird behaves completely as belonging to the Baltic population (Table 6). The Eiders ringed ad ducklings show the first two calendar years a certain amount of dispersion, mainly into a SW direction. The farthest recovery was 1,170 km away from the place of birth. However, again a considerable part of the immature birds tend to winter in the Wadden Sea. After reaching maturity the distribution of the ringed ducklings is clearly different between sexes. From 113 controls of nest-caught ducks and from recoveries it appears that, with very few exceptions, the females nest in or near their place of birth (Fig. 13). In contrast, there are very few recoveries from the drakes during the breeding season near their place of birth. With 44 recoveries from abroad against only 23 from the Wadden area their pattern of distribution is clearly different from that of the ducks. Since Finland gives already 32 recoveries during the breeding season it is clear that the majority of the drakes have no particular bond with the colony, in which they hatched (Fig. 14). This behaviour of young drakes, in combination with a tendency to abmigration of older drakes, wil1 promote genetic exchange between colonies in widely different places. In this connection the statement by Milne & Robertson (1965) that the Ythan estuary in Scotland is inhabited by a group of Eiders showing genetic isolation, differing from neighbouring birds in migrational behaviour and in the composition of protein in the eggs, needs further confirmation. In the breeding season numerous immature birds spend the summer near the Dutch colonies. Their numbers are often the same or exceed the number of breeding birds. Part of them are local young birds, but the majority comes from elsewhere. Ringing recoveries point to a Baltic origin of these birds (Figs. 7 and 10). The total number of Eiders, immatures and adults during the breeding season amounts to 20,000-30,000. In this period they stay near the coast (Fig. 16), some visit the shore at high tide. During moulting they are less tied to the shore (Fig. 17), while the total number of birds seems to increase slightly. Four recoveries in August and September from mature ducks ringed in the Baltic area may point to a moulting migration to the Dutch waters (Fig. 15, Table 10). Moreover our own breeding birds and immature summer visitors moult in the Dutch Wadden Sea (Fig. 8, 11, Table 7). In winter the Eider clearly avoids the coast (Fig. 18), only immature and sick birds keep near the shore. The numbers at that time exceed 100,000, a multiple of numbers in summer. Besides the native breeding birds, the summer visitors and moulting birds there are numerous migrants arriving from abroad. Ringing recoveries point to the Baltic area as the home of those winter visitors (Fig. 15, Tables 8, 9, 10). There is no reason to assume a recent change in behaviour as mentioned by Bauer & Glutz (1969) from the fact that the local breeding population winters in the Wadden Sea. Already in the previous century the Eider was a numerous winter visitor, while summer visitors were observed as early as 1863. The statement by Taverner (1959, 1963, 1967) that the birds wintering off the shore of Southern England are mainly Dutch breeding birds, is probably based on an incorrect interpretation of ringing and counting data. Since the birds off the English coasts are mainly immature birds they can hardly come from the Netherlands, as the survival of chicks in our colonies is very low (Swennen, in prep.). Most of them wil1 be offspring of the Baltic Sea population which, according to Almkwist et al. (1975) comprises 297,000 pairs. In comparison, the total Dutch breeding population is with about 4000 pairs very small. If only +% of the Baltic immatures wander slightly beyond Danish, German and Dutch waters, the winter visitors in coastal waters of Southern England could be accounted for. The diets of mature Eiders in the Dutch Wadden Sea was studied by direct observations, by examining stomachs of shot birds (Table 13, 14, Fig. 19), birds found drowned in fishing nets (Table IS), birds found dead, and by investigating spontaneous vomits, vomits of fleeing birds, and faeces. Some of these methods gave insufficient or too selective information, others met with practical difficulties. The faeces gave a good picture of the preys eaten. Fresh faeces could be collected over the whole area during low tide crossing by ship or rubber boat to the banks and tidal flats where the Eiders rested. The average weight of the faeces collected per specimen corresponded with the average weight of the indigestible parts from the food in the shot birds. Therefore the droppings of a single bird, whatever the volume might be, were considered to represent one 'meal'. It appeared that most of the meals consisted of one prey species only. If they were mixed, estimation was made of the respective flesh volume. Meals consisting of more than 3 prey species were booked as 'mixed'. Species representing less than 10% of a meal were left out in the final composition. If they should have been included the list of prey species would have been extended with algae, hydroids, barnacles and other epibionts of the real prey species, not representing a real contribution to the energy requirement of the Eiders. Samples were collected in areas which held at least 500 Eiders. During 3 years 4,441 Eider 'meals' have been collected in various places in the Wadden Sea. No clear seasonal variation in the main food components was found. Molluscs predominate; crustaceans take the second place (Table 16). In agreement with their occurrence in the Wadden Sea Cardium edule and Mytilus edulis each comprise 40% of the diet. The part of 6%, provided by Carcinus maenas, is more than the part this species contributes to the total biomass of the Wadden Sea benthos, according to Beukema (1976). This may point to a preference for crabs by the Eider. Observations on mussel-beds show that the mussels taken by Eiders are well below the average size. Captive Eiders, presented with cockles, also showed that the birds select the smaller sizes, though shells measuring 49 mm could still be swallowed by both sexes (Fig. 20). It also appeared that selection took place against irregular shell-surfaces (Table 17). This may be due to the risk connected with swallowing big, hard or sharp objects. Indeed three dead Eiders showed severe injuries of the stomach wall due to hard remains of a crab and mussels, respectively, and Cottam (1939) published a photograph of an Eider in which a slightly too-big mussel had got stuck between tongue and palate. In the Wadden Sea feeding takes place during short periods alternating with preening and resting periods of 10-20 minutes. Moreover there are longer resting periods of 1+-5 (10) hours, often coinciding with high or low tide. In some cases the birds were observed to feed during the night. Feeding activity of birds in captivity was registered by means of a photo-electric cell. The start of daily feeding activity shifted from half an hour before sunrise in January to half an. hour after sunrise in June. The end of the feeding period was much more irregular than the beginning. Often the animals went on till long after sunset, sometimes they stopped in the afternoon and fed again during some hours in the night. There was no connection between nocturnal activity and moon phase. After a good deal of nightly feeding activity there used to be longer resting periods during the day. In winter the part played by nocturnal activity was greater than in summer with the result that the average feeding time per 24 hours is more or less the same (Table 16). The rate at which undigested parts of various preys pass through the digestive tract was found to be slightly over an hour (Table 19). One bird produced the first faeces 58 minutes after filling the gullet, after 86 minutes faeces production stopped. Then the amount of excreted shell fragments was already practically the same as the weight of the shells the bird had eaten. Since marine molluscs are swallowed whole, it means that the alimentary system has to deal with a considerable amount of superfluous matter. For one swallowed cockle about half the weight is water, slightly less than half the weight is shell, 1 to 2% is salt and only 2-3% is flesh (Table 21). Considering the great differences in condition index or flesh volume of the molluscs in the area between winter and summer (Table 21, Beukema 1975) it is remarkable to find that the largest numbers of Eiders visit the area during the period that the condition of the molluscs is lowest. Therefore the largest total consumption coincides with the period in which the birds have to eat twice the number of prey animals to arrive at the same flesh consumption as during summer. The daily food consumption was determined with captive Eiders. These birds were kept for a number of years in an aviary, the bottom of which consisted of a basin measuring 4 9 0.6 m and filled with running seawater. The food given was natural food collected in the Wadden Sea. In 273 experiments, lasting 24 hours, the consumption of cockles was determined exactly. Daily variation proved to be considerable. There was no clear connection with either the weight of the birds, the season and the temperature, the latter varying between 1 and 17ŚC. On average the daily consumption was found to be 106.8 g cockle-flesh (ash-free dry weight), with an average weight of the birds of 2,200 g (Table 22). Assuming an extra 30% food uptake in free-living birds, more exposed to wind, waves and tides and searching actively for food, the food uptake of the Eider would be 138 g ash-free dry weight of 750 Kcal per bird per day. The average number of Eiders in the Dutch Wadden Sea will be about 63,000. Their total annual consumption will therefore be 63,000 x 365 x 138 g dry flesh (or 720 Kcal) = 3.2 x 106 kg or 16.5 x 109 Kcal. This is about Y, of the consumption of all carnivorous birds in the area, which is estimated at 10 x 106 kg or 52x 109 Kcal (Swennen 1974, 1975). For the whole Dutch Wadden Sea this works out at an average consumption by the Eider of 1.2g ash free dry weight or 6.2 Kcal per m2 per year. This is in the same order of magnitude as the food consumption of the Shore Crab Carcinus maenas in this area (Klein Breteler 1976).