A longitudinal study of the effects of trees, geese and avian predators on breeding wader meadow birds: the case of the Demmerik polder, the Netherlands

In the Netherlands, breeding populations of wader meadow birds are in sharp decline. One possible cause is that breeding areas are becoming less open because of tall trees and other factors. The effects of tree lines are generally studied by means of transversal studies spanning a short period of time. We report on a longitudinal field study from 1993 to 2010 into the breeding densities of Eurasian Oystercatcher (Haematopus ostralegus), Northern Lapwing (Vanellus vanellus), Black-tailed Godwit (Limosa limosa) and Common Redshank (Tringa totanus) in the Demmerik polder, the Netherlands. One part of this polder, a nature reserve, has an older tree line, while two agricultural parts are divided by a newly developing tree line. As the tree line in the agricultural parts was developing, foraging families of Greylag Goose (Anser anser) showed a strong increase in only one of these parts during the breeding season. During the same period, the density of avian predators also increased in the whole polder. Analysis shows waders avoid trees in both situations: with an existing tree line (a static situation) and with a growing tree line (a dynamic situation). We investigated the possible role of geese and avian predators in explaining the decline in density of breeding wader meadow birds, by systematic comparison of several different models. In these models, the effect of the growing tree line has the greatest impact on breeding meadow birds. Models with geese describe the trends of breeding wader meadow birds better than those including avian predators, but since these two variables, geese and avian predators, are confounded, no definitive conclusion can yet be drawn. Potential explanations of and functional mechanisms behind the strong decline in breeding meadow bird populations in this area are discussed.