Guest blog by Nino Maag, postdoctoral researcher at the Swiss Ornithological Institute SOI. This research was a collaborative effort of the SOI, RSPB, Museum and Institute of Zoology – Polish Academy of Sciences, UK Centre for Ecology & Hydrology, Laboratory of Forest Biology – Wrocław University, Department of Biology – University of Marburg, and Division of Conservation Biology – University of Bern.
Populations of the wood warbler Phylloscopus sibilatrix are declining in many but not all regions of Europe. In general, wood warbler numbers have been declining at the western edge of their breeding range, including the UK, but have been stable in some regions of central Europe, for instance in Poland and Russia. The reasons for the declines are largely unknown.
The wood warbler in its natural habitat © Michael Gerber
In this collaboration, we zoomed in to six local populations in the UK (mid-Wales, Dartmoor, the New Forest), Germany (Hessen), Switzerland (Jura mountains), and Poland (Białowieża National Park), had a closer look at the local trends of the past 15 years, and investigated if reproductive success and nest predation during the same period were in line with these trends.
Wood warblers breed on the ground of broadleaved or mixed conifer-broadleaved forests © Lukas Linder, Jura, Switzerland.
Reproductive success (i.e., average number of fledglings per nest) was lowest in the New Forest (1.43), intermediate in Jura (2.41) and Białowieża (2.26), and highest in mid-Wales (3.02) and Dartmoor (2.92).
The figure below shows that high reproductive success did not generally translate to positive population trends. For example, Dartmoor had negative population trends despite high reproductive success. However, populations with lowest reproductive success, for example the New Forest and Białowieża, had negative trends.
Relationship between local wood warbler population trends and reproductive success in the past 15 years. DM=Dartmoor, JR=Jura, BW=Białowieża, NF=New Forest, MW=Mid-Wales, HE=Hessen.
The reason for low reproductive success in the New Forest, Jura and Białowieża was low nest survival. High reproductive success in mid-Wales and Dartmoor was due to high nest survival and large clutch sizes. Predation rates were similar everywhere, but the dominant nest predator classes differed between the regions: nests in mid-Wales were mainly predated by birds; in Dartmoor, the New Forest, Hessen, and Jura similarly by birds and mammals; and in Białowieża exclusively by mammals (see below).
Causes of nest failure in the six study regions: Mid-Wales, Dartmoor, New Forest, Hessen, Jura, Białowieża. Other failures include desertion, trampling, etc. Circle sizes refer to local sample sizes.
The most common avian predators of wood warbler nests were jays, buzzards, and sparrowhawks; the most common mammalian predators were pine martens, foxes, and badgers. Nest predators were recorded with a total of 645 nest cameras deployed at 1-2 m from the nest. Below, a pine marten can be seen in front of a depredated wood warbler nest at the Białowieża study site. Because they are very rare in the UK, martens were not recorded by cameras. The nest predator most observed in the UK was the jay.
Pine marten captured by a Reconyx trail camera. Deployed by Marta Maziarz, Białowieża National Park, Poland.
Predation rates of wood warbler nests by these different predator species were related to different habitat features. For example, nest predation by buzzard and pine marten was higher if habitat fragmentation was high, and nest predation by badger was higher if there was grassland rather than arable land next to the forest. We suggest that predator-specific nest predation rates and their association with habitat features should be considered when making management decisions.
More details can be read in two open access studies recently published:
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