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

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

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

De vangst van sterns en het verwijderen van ringen in Afrika: een probleem = The problem of tern catches and the removal of rings in Africa

In the West African winter quarters the practice of catching European terns has traditionally been widespread. Birds are caught for food or for pleasure. Although in some countries measures have been taken to discourage these practices and the incidence of catching terns seems to have decreased in Ghana, recent research indicates that terns may sometimes be caught to obtain the rings as a trophy. Some birds appear to have been released after the ring has been removed. An analysis of ringing recoveries of Dutch Sandwich Terns was carried out, revealing that of 48 ringed birds captured, at least ten birds were released alive after removal of the ring, while 14 birds were released with or without ring and of 14 more individuals nothing was recorded (Table 1). Thus, the analysis of ringing recoveries of European terns may not be useful to determine mortality rates. A further analysis of the scope of this problem seems to be necessary

Tern trapping along the Senegalese coast = De vangst van sterns in Senegal

This paper reports the trapping of terns along the Senegalese coast in the 1990s. In spite of earlier education programmes, trapping of terns still seems a common habit in Senegal. During the winters of 1996 and 1997, a total of 210 rings was found, mainly in bracelets and necklaces. About 80% of the rings originated from the British lsles, The Netherlands and Belgium. The species most affected was the Sandwich Tern. It is estimated that at least 500 Sandwich Terns and an additional 500 terns of other species are annually caught along the coastline of Senegal. This estimate, however, is extremely optimistic. The actual number of terns trapped in Senegal is probably much higher, possibly even a tenfold of this. Tern catching is still believed to be an important mortality factor for these vulnerable species

Very short-lived bromomethanes measured by the CARIBIC observatory over the North Atlantic, Africa and Southeast Asia during 2009-2013

Short-lived organic brominated compounds make up a significant part of the organic bromine budget in the atmosphere. Emissions of these compounds are highly variable and there are limited measurements, particularly in the extra-tropical upper troposphere/lower stratosphere and tropical troposphere. Measurements of five very short-lived bromomethanes (VSLB) were made in air samples collected on the CARIBIC project aircraft over three flight routes; Germany to Venezuela/Columbia during 2009-2011, Germany to South Africa during 2010 and 2011 and Germany to Thailand/Kuala Lumpur, Malaysia during 2012 and 2013. In the tropical troposphere, as the most important entrance region to the stratosphere, we observe a total mean organic bromine derived from these compounds across all flights at 10-12 km altitude of 3.4 ± 1.5 ppt. Individual mean tropical tropospheric mixing ratios across all flights were 0.43, 0.74, 0.14, 0.23 and 0.11 ppt for CHBr3, CH2Br2, CHBr2Cl, CHBrCl2 and CH2BrCl respectively. The highest levels of VSLB-derived bromine (4.20 ± 0.56 ppt) were observed in flights between Bangkok and Kuala Lumpur indicating that the South China Sea is an important source region for these compounds. Across all routes, CHBr and CHBr2 accounted for 34% (4.7-71) and 48% (14-73) respectively of total bromine derived from the analysed VSLB in the tropical mid-upper troposphere totalling 82% (54-89). In samples collected between Germany and Venezuela/Columbia, we find decreasing mean mixing ratios with increasing potential temperature in the extra-tropics. Tropical mean mixing ratios are higher than extra-tropical values between 340-350 K indicating that rapid uplift is important in determining mixing ratios in the lower tropical tropopause layer in the West Atlantic tropics. O3 was used as a tracer for stratospherically influenced air and we detect rapidly decreasing mixing ratios for all VSLB above ∼100 ppb O3 corresponding to the extra-tropical tropopause layer

Grote sterns <i>Sterna sandvicensis</i> verwijderen eischalen en dode kuikens uit de kolonie = Sandwich terns remove egg shells and dead chicks from the colony

Observations of Sandwich Terns removing eggshell fragments are reported. On one occasion a dead chick was seen to be carried away from the colony. This is not a common behaviour in Sandwich Terns, but it seems to be functional because eggshells and dead chicks may attract predators and may be a source of infections