BOTH C. VISSER ME & BALEN H. VAN (2002) The rise and fall of a Tree Sparrow population. LIMOSA 75 (2): 41-50.

Eurasian Tree Sparrows established themselves as breeding birds in nestboxes in the Hoge Veluwe area in 1966, and the population grew strongly during the next 15 years. In the early eighties population numbers crashed in just a few years, and have not recovered since. In this paper we aim to explain the demographic causes of this observed population growth and crash. Population growth rate was strongly related to the annual number of fledged offspring per pair, and the four years in which reproductive success was experimentally reduced, support the idea that the observed population fluctuations are a result of variation in the number of fledged offspring. During the population growth phase, the reproductive success of first broods was equal to that in the period of decline, but the probability to produce a second and third brood as well as their success were higher during population growth than after the crash. The population crash coincided with three years in which reproductive success was experimentally reduced. In a simple simulation model we showed that the population dynamics could be the result of the observed fluctuations in reproductive success, without any additional change in mortality, suggesting that reproduction and not mortality is the cause of the observed dynamics. We consider different hypotheses that can account for the observed rise and fall of this Eurasian Tree Sparrow population. Intensification of agriculture and its subsequent reduction in seeds seems not important because that would mostly affect survival and not so much reproduction. Adverse effects of pesticides on reproduction were considered unlikely, since the sparrows increased during the period when the most persistent pesticides were used, and the effect was not found on the first broods. An increase in Sparrowhawk predation can in principle explain both the decrease and the increase of the Eurasian Tree Sparrows, but our results point in the direction of reproduction, and not mortality, being the most important factor driving the population dynamics. We discuss the possibilities that insect food abundance during the late breeding season may have declined, or that diseases or parasites have increased. Our results are in contrast to what happened in Britain, where the Eurasian Tree Sparrows probably declined as a result of decreased survival, and where reproduction increased simultaneously. Over de oorzaak van de afname doen verschillende theorieën de ronde. Intensivering van de landbouw zou leiden tot een lager voedselaanbod waardoor de reproductie daalt en/of de sterfte stijgt (Summers-Smith 1995). Vooral de afname van (winter)graanteelt in grote delen van de Nederlandse zandgronden zou deze zaadeter parten spelen. Ophoping van landbouwgif in de vogels is ook gesuggereerd, en het daaruit voortvloeiende negatieve effect op reproductie en sterfte (Wesokowski 1991). Tenslotte zou de toegenomen predatie door roofvogels, en dan met name de Sperwer Accipiter nisus, deze relatief makkelijk te vangen prooisoort in aantal kunnen reduceren (Summers-Smith 1995). In dit artikel geven we een beeld van de aantalsontwikkeling van de ringmuspopulatie op de zuidelijke Hoge Veluwe. We analyseren of deze aantalsontwikkeling samenhangt met veranderingen in het reproductief succes. Tenslotte proberen we op grond van deze gegevens te komen tot een idee over de ecologische oorzaak van de afname in deze populatie.

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