Effects of climate change on phenology in the willow warbler

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Effects of climate change on phenology in the willow warbler

In this study on Sweden’s most common bird species the willow warbler, Phylloscopus trochilus (Swedish: lövsångare), we want to investigate how phenological events have changed in the last 20 years and achieve a holistic analysis of breeding area phenology, including both spring and autumn migration, as well as the timing of reproduction.

Willow warbler. Photo: Johanna Hedlund

Willow warbler (Phylloscopus trochilus) male colour ringed on Gotland, 2011. Photo: Johanna Hedlund.

Nature changes. It is only natural. Life on earth adapts to the alterations, succeeding or failing. But what happens when nature changes very rapidly, and without halt? The effects of the recent global climate are more rapid and continuous than any other changes seen during recent eras. For the past century, the globe has seen an average temperature rise of 0.8°C, with the fastest change being initiated in the late 1970s and reaching an increase of almost 0.2°C per decade since.

Climate is an ever-present dynamism in the natural world. It affects the biota directly and long-term, and reports on behavioural and ecological responses in fauna and flora to climate change are accumulating. Especially migratory patterns have received a large fraction of attention in climate research on birds. Migrants experience a great number of different conditions and habitats and are as such more vulnerable. This makes them exemplary study species in a climate research context.

Timing is everything in a dynamic world and migration is an important part of the life-history clockwork of many bird species. An estimation of 5 billion birds leave the Palearctic for Africa each year making the Euroafrican flyway one of the largest animal movement in the world. In recent years, this phenological mass-event is beginning to shift. Reports on changes in migration and life-history traits correlating with climate change effects are accumulating from all continents. In Europe the dominant trend appears to be an advancement of spring arrival and of egg-laying dates, presumably made possible by improved conditions earlier in spring. However, studies report differing results on the effects of the same climate variables and responses vary between species and also within species, the cause of which is yet to be better understood. Another difficulty in interpreting results is the lack of more holistic investigations, i.e. including more than one phenological event in the analysis.

Willow warbler. Photo: Johanna Hedlund

Willow warbler (Phylloscopus trochilus) female at her nest, Abisko 2011. Photo: Johanna Hedlund

In this study, part of the research project of PhD student Johanna Hedlund running 2011-2015, Sweden’s most common bird species the willow warbler (lövsångare in Swedish) has been chosen as a study animal for the investigation of climate change effects on avian phenology. We want to investigate how phenological events have changed in the last 20 years and achieve a holistic analysis of breeding area phenology by including both spring and autumn migration, as well as the timing of reproduction. We also aim to study several populations at different latitudes and within-species differences in phenological response.

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