Settlement behaviour, predation and population fluctuations in the wood warbler

The wood warbler is endangered in Switzerland and in many parts of Europe. A research project of the Swiss Ornithological Institute examines which factors affect the strong fluctuations in population size.

Aims

Since the nineties, populations of the wood warbler (Phylloscopus sibilatrix) have shown different trends throughout the European range: in Eastern Europe populations strongly fluctuate from year to year, while in Western Europe populations are steeply declining in many areas. The reasons for these contrasting population trends are not clear.

An increase in predation and changes in forest structure are primarily discussed as possible reasons for the decline in Western Europe. Since the decline and eradication, respectively, of rabies, red fox populations have strongly increased – also in forests. The associated increased predation pressure could indeed have led to a decline in breeding success of this ground-nesting species and hence to the observed population decline. However, recent studies also suggest changes in forest structure due to altered forestry activities as a further possible reason for the negative population trends. To what extent increasing recreational activities in forests and the thereby caused disturbances, food shortages due to climate change as well as habitat change and destruction in the stopover sites and wintering areas have an impact on population declines is subject to controversial discussion.

The project addresses the following aspects:

1. Examination of the settlement behaviour of wood warblers. The role of selected cues for settlement decisions will be addressed. Examples of such cues include the presence of rodents (i.e. mice) and predators, the presence of conspecifics (social information), own reproductive success in the previous year (personal information), food availability, etc. The influence of habitat factors on settlement will be considered as well.
2. Assessment of spatial and temporal variation in reproductive performance. From the perspective of species conservation, it is important to know the factors affecting nest success and fledgling numbers.
3. Investigation into the interactions among wood warblers, predators and the availability of beechnuts and acorns. Studies from North America suggest that mast years (i.e. years with mass fructification) have a substantial effect on rodent density in the next spring, which in turn affects rodent-hunting predators. These predators influence reproductive success of ground-nesting passerines. Such interactions could also occur in deciduous forests in Europe and affect population size.

Approach

The studies mainly take place in forests of the Swiss lowlands and the Jura Mountains. Some aspects are investigated close to Marburg, Germany, and in the primeval forest of Białowieza, Poland. Central elements of the field methods include the search for territories and breeding pairs, the use of trail cameras to monitor nests, the measurement of habitat characteristics and rodent density (primarily Apodemus and Myodes species). Details on the methods can be found in the papers listed under Publications.

Significance

In 2010 the wood warbler was added to the Red List of breeding birds (classified as “vulnerable”) and therefore belongs to the threatened and endangered species of Switzerland. Moreover, the wood warbler is one of 50 priority species of the Swiss species recovery programme for birds that the Swiss Ornithological Institute and the Swiss Association for the Protection of Birds SVS/BirdLife Switzerland are conducting in close collaboration with the Federal Office for the Environment FOEN. The reasons for the population declines in many areas of Western Europe are not clear. Within the frame of a species recovery project, we try to improve the habitat of the wood warbler with targeted conservation measures.

Results

A number of MSc theses and one PhD thesis have revealed the following:

1) Settlement of the wood warbler is influenced by environmental factors and the presence of conspecifics. The species prefers to settle along steep slopes covered by middle-aged mixed deciduous forests with nearly closed canopy, low vertical structural diversity and a relatively open under- and mid-storey. The forest floor is covered by sparse ground vegetation, which includes tussocks and grasses typically favoured by females for nest placement. In addition, forest parts with few rodents are preferred for territory settlement. Across years, population size of wood warblers is inversely related to the abundance of rodents: in years with few rodents, many wood warblers settle, while in years with many rodents, the species is hard to find.

2) Data from Switzerland, Germany and Poland show that approx. half of the nests are successful in fledgling young. The main cause of nest loss is predation, carried out mainly by marten and Eurasian jay. The red fox only plays a minor role as nest predator, rodents hardly any at all.

3) In rodent-outbreak years, nest success is lower than in other years. This appears to be related to the predators of rodents, because more nest losses are due to mammals like marten and fox in rodent-outbreak years compared to other years. Moreover, nest survival increases with camouflage of the nest and number of grass tussocks surrounding the nest.

4) Currently occupied territories are located farther from the forest edge and in steeper slopes than previously occupied, but today abandoned territories. Furthermore, air nitrogen deposition was significantly larger in areas abandoned over the past 20 years compared to areas still occupied and newly colonized, respectively, by the wood warbler. Forest management also appears at least partly to explain the decline of the species, as territories still occupied today have more trees and are located farther away from forest clearings compared to abandoned territories.

5) In recent years, some forest bird species seem to miss the spring caterpillar peak which has lately advanced owing to the warming climate. These birds do not seem to be able to advance their breeding onset at the same pace and thus miss the caterpillar peak at the time when the nestlings have the greatest food demands. Caterpillars are a key food source for raising nestlings in many forest song birds, and this “food mismatch” has been linked to population declines in some species. However, no evidence for a food mismatch has so far been found in wood warblers. These birds appear to be able to successfully raise their broods with foods other than caterpillars.

Project management

Gilberto Pasinelli, Alex Grendelmeier

Partners

Raphael Arlettaz, Conservation Biology, University of Bern
Christian Ginzler and Felix Kienast, Swiss Federal Institute for Forest, Snow and Landscape Research WSL
Tomasz Wesołowski and Marta Maziarz, Laboratory of Forest Biology, Wroclaw University 
Dana Schabo and Nina Farwig, Conservation Ecology, Marburg University

Financial support

Hilfsfonds für die Schweizerische Vogelwarte Sempach
Stotzer-Kästli-Stiftung
Styner-Stiftung
Lotteriefonds des Kantons Solothurn
Swiss National Science Foundation
PD Stiftung der Universität Zürich
Basler Stiftung für biologische Forschung
Emilia Guggenheim-Schnurr-Stiftung
Carl Burger-Stiftung Münchenstein

Publications