Living with gulls: trading off food and predation in the Sandwich Tern <i>Sterna sandvicensis</i>

The low-lying, sandy areas along the Dutch coast offer important breeding opportunities for Sandwich Terns Sterna sandvicensis. Throughout the twentieth century Sandwich Terns nested here in fluctuating numbers. The Dutch population suffered from a major kill in the 1960s due to pesticide pollution causing the number of breeding pairs to drop from over 35,000 in the 1950s to 875 in 1965. After the spill of pesticides had stopped the numbers slowly increased but after 40 years the population has not yet fully recovered. The slow and incomplete recovery of the Dutch population was a source of concern and the present study aimed at a better understanding of the factors regulating the size of the Dutch Sandwich Tern population. Following the crash in the 1960s, the size of the Dutch population positively correlated with the amount of young herring present in the North Sea (Brenninkmeijer and Stienen, 1994). This relationship suggests that the Dutch Sandwich Tern population is limited by food availability. For this reason we concentrated the study on the feeding ecology of Sandwich Terns, hoping to find links with population dynamics

Pilot study on the influence of feeding conditions at the North Sea on the breeding results of the Sandwich Tern <i>Sterna sandvicensis</i>

Of all gulls and terns breeding along the Dutch coast the Sandwich Tern is the most specialised piscivorous bird. During the breeding season terns feed mainly on pelagic fish as herring, sprat, sandeel, and lesser sandeel. This makes the species extremely vulnerable to changes in the marine ecosystem.During the twentieth century, the number of breeding Sandwich Terns in the Netherlands has shown large fluctuations. In the beginning of this century, numbers were low, because of large-scale egg-collecting and shooting of adults. As a result of the protection following the slaughter for the lady's hat fashion, numbers gradually increased up to 45,000 pairs in 1938. In World War II, egging again caused a drop in numbers. After World War 11, the population increased up to 36,000 pairs in the 1950s. At the end of that decade, numbers dropped markedly and the number of breeding pairs were reduced to 875 in 1965. This decrease was due to a pollution of the Dutch coastal waters by organochlorine pesticides. After the pollution stopped, the population increased in numbers again, but the large numbers from the period previous to the pollution have never been reached since then. Compared to previous population recoveries, the recovery after the pollution in the 1960s was slow, and since the 1 980s the population seems to stabilise at approximately 11,000 breeding pairs, almost one third of the population during the 1930s and the 1950s.This leads to the main subject of this report: 'What is the reason of the slow recovery of the Sandwich Tern after the numerical decline in the 1960s and why is there a stabilisation of breeding pairs at such a low level?’. Although this could be due to many factors, we have focussed our study on the food situation during the chick rearing period, The study deals with three items:( 1 ) What is the species composition and what is the length distribution of prey items delivered to the chicks of Sandwich Terns on Griend, the main colony in The Netherlands, and have there been remarkable shifts compared with the late 1960s and early 1970s (chapter 2)?(2) Where do the terns feed, and are there any shifts in this respect compared with the late 1 960s and early 1970s (chapter 3)?(3) Is there a relationship between fish availability and either population size or breeding success of the Sandwich Tern (chapter 4)?The species composition of the fish delivered to the chicks in 1992-1993 was identical to that in 1969-1974. The number of fish brougth to the chicks, however differed markedly from that in 1969-1974. In bath years (mast pronounced in 1992) the food supply was law compared with 1969-1974, and also compared with foreign colonies. The amount of food actually eaten by the chicks was strongly influenced by windspeed. An increasing windspeed did not only lower the supply of food (resulting from a lower foraging success) but also increased kleptoparasitism by Black- headed Gulls. The low food supply in recent times might be a possible explanation for the stabilisation of the number of breeding pairs since the early 19805.Experiments with radio-tagged adults revealed that the foraging area of the terns is not markedly different from that in 1970, although there a little shift towards the Wadden Sea.Analyses of data on the abundance of young herring in the North Sea and the number of breeding Sandwich Terns revealed a positive relationship between these two parameters. However, the number of fledged young showed no relationship with the abundance of young herring. Although the data set on young herring refers to the total North Sea and do not necessarily show the local abundance of this species, the results suggest that the breeding population of the Sandwich Tern at Griend is regulated by the availability of food

Eierrovende Lepelaar <i>Platalea leucorodia</i> op Griend

We observed an adult Spoonbill in full breeding plumage taking Black-headed Gull Larus ridibundus eggs on the island of Griend, Dutch Wadden Sea, on 13 May 1992. Five or six times, the Spoonbill took an egg from two gull nests within five minutes. Eggs were taken in the bill, then thrown up, and caught in the open bill held up vertically. In the process, the bird was constantly and intensely attacked by gulls; in response, it drew in its neck and raised its head feathers. After this successful robbery the bird tried to take eggs in another subcolony. This time the bird was not successful, probably due to heavy gull attacks. A few hours later, another Spoonbill in breeding plumage tried to take eggs from Black-headed Gull nests on the island, but without success

Impact of wind turbines on birds in Zeebrugge (Belgium)

We studied the impact of a wind farm (line of 25 small to medium sized turbines) on birds at the eastern port breakwater in Zeebrugge, Belgium, with special attention to the nearby breeding colony of Common Tern Sterna hirundo, Sandwich Tern Sterna sandvicensis and Little Tern Sterna albifrons. With the data of found collision fatalities under the wind turbines, and the correction factors for available search area, search efficiency and scavenging, we calculated that during the breeding seasons in 2004 and 2005, about 168 resp. 161 terns collided with the wind turbines located on the eastern port breakwater close to the breeding colony, mainly Common Terns and Sandwich Terns. The mean number of terns killed in 2004 and 2005 was 6.7 per turbine per year for the whole wind farm, and 11.2 resp. 10.8 per turbine per year for the line of 14 turbines on the sea-directed breakwater close to the breeding colony. The mean number of collision fatalities when including other species (mainly gulls) in 2004 and 2005 was 20.9 resp. 19.1 per turbine per year for the whole wind farm and 34.3 resp. 27.6 per turbine per year for 14 turbines on the sea-directed breakwater. The collision probability for Common Terns crossing the line of wind turbines amounted 0.110-0.118% for flights at rotor height and 0.007-0.030% for all flights. For Sandwich Tern this probability was 0.046-0.088% for flights at rotor height and 0.005-0.006% for all flights. The breeding terns were almost not disturbed by the wind turbines, but the relative large number of tern fatalities was determined as a significant negative impact on the breeding colony at the eastern port breakwater (additional mortality of 3.0-4.4% for Common Tern, 1.8-6.7% for Little Tern and 0.6-0.7% for Sandwich Tern). We recommend that there should be precautionary avoidance of constructing wind turbines close to any important breeding colony of terns or gulls, nor should artificial breeding sites be constructed near wind turbines, especially not within the frequent foraging flight paths

Voedsel en groei van kuikens van de Velduil <i>Asio flammeus</i> op Griend

One pair of Short-eared Owls nested on Griend, an islet in the Dutch Wadden Sea, in 1992, 1994, 1995 and 1996. The species never bred on Griend prior to the 1990s. In the four years one, one, four and zero chicks fledged. Growth of the chicks showed differences corresponding with hatching order. Oldest chicks grew faster than their younger siblings. In 1996, chicks grew notably slower than in 1995. Pellets and plucking remains found in the immediate vicinity of the chicks revealed that Dunlins and Wood Mice made up 69 and 28%, respectively, of prey items in 1995. Other avian prey included Redshank, Turnstone, Common- and Arctic Tern. In 1996, however, Wood Mice made up 76% of prey items, while Dunlins accounted for only 8%. Timing of the start of the breeding season seems important for the Short-eared Owl on Griend. It is suggested that the islands' Wood Mice population alone does not allow for optimal growth and survival of owl chicks. When owls started breeding relatively early (1995: late March), Dunlins, present in large flocks, constituted an additional prey during the chick rearing period. This resulted in fast chick growth and good breeding success. In 1992 and 1996, Dunlins disappeared to migrate to their breeding areas one week after owl eggs had hatched; consequently, chick survival was low. Wood Mice were inadvertently introduced to the island in 1988