How Climate Change Alters the Interactions Among European Birds

Note: Recently, my research paper about the effect of climate change on three species of European birds was accepted by the IOP Conference Series: Earth and Environmental Science (EES). It is set to be presented at an international conference. Rather than having it buried in the vast archive of papers, I think it would be much more productive for me (and for potential audience) to explain my findings in a post.

Global warming is advancing the timing of spring – trees grow leaves and flowers bloom earlier than before. Some birds advance their timing of egg laying to track the advancement of the time when natural resources are at a maximum. But not all birds have the same adaptive ability. For example, birds that stay in the same area should, in theory, be better adjusted to changes in local environment than do migratory birds.

Through synthesizing long-term data obtained from four previously published studies, I find that different species of European birds adjust the timing of egg laying at a different pace. These variable responses could have an impact on the interactions between these species, such as their competition for nests and the occurrence of hybridization.

Unexpectedly, even within the same species, birds in different geographical locations advance egg laying at a different rate. As a result, the effects of climate change on interactions between species can vary in different geographical locations.

Competitive relationship is altered

Pied flycatchers are long-distance migratory birds that overwinter in Western Africa and disperse throughout Europe during the breeding season. Great tits are one of the most common resident birds in Europe. These two species actively compete for nest holes.

In general, the tits occupy the nest holes for quite some time in advance of the migratory flycatchers. The flycatchers then seize nesting holes from the tits in two ways: intense attacks against a great tit when it flies to and from the nest, resulting in nest desertion by the tit; or rapid nest-building in a hole already occupied by a tit when the tit is foraging during the day. But once the incubation period of great tits starts, takeover attempts are often fatal for the flycatchers. So the earlier flycatchers could start egg laying in comparison with the tits, the more advantaged they are in the competition.

In study sites across Central Europe, the pied flycatcher advanced their mean laying dates (MLD) significantly faster than the tits, as shown in Figure 1. The timing of egg laying for the flycatchers is, on average, “catching up” with that for the tits. As a result, the flycatchers are likely to be favoured in the competition for nest holes.

In study sites across Western Europe, the pattern is much more varied and complex, as shown in Figure 2. At some locations, the flycatchers advanced their MLDs faster than the tits, whereas the trend reverses in other locations. Therefore, the flycatchers are likely to gain an advantage only in a few study sites, but not in others.

Hybridization frequency may be affected

Collared flycatchers are migratory birds closely related to the pied flycatcher. They share overlapping ranges in Central Europe and on the Gotland island in the Baltic, where they hybridize regularly.  

In Northern Europe, my analysis shows that pied and collared flycatchers shared a smaller overlap in breeding dates as a result of asymmetrical response to climate change. This may reduce the temporal coincidence of mate choice between the two closely related species, hence decreasing the occurrence of hybridization. Less frequency of hybridization benefits both species mainly because hybrid individuals hatch fewer eggs and therefore fledge fewer offspring. 

In Central Europe, the two flycatchers potentially shared a great overlap in breeding dates. It is perhaps noteworthy that a comparison between the pied flycatcher population in southern Germany and collared flycatcher populations in the Czech Republic reveals that the difference in mean laying dates diminished by several days from 1980 to 2000. As a result, the frequency of hybridization is expected to increase.

Geographical variation calls for local studies

The results highlight that geographical variations in the advancement of laying dates have complicated and variable effects on interactions between different species, a novel research field that lacks empirical results. 

In the future, field research should be conducted in order to provide empirical evidence for the effects outlined in this study. Future studies on climate change and its effects on ecological communities should be replicated in different locations in order to account for geographical variability.


Acknowledgement

I thank André Dhondt and Yuting Qian for guidance and useful discussions. I am further grateful to Camille Dolmont for comments on the manuscript, as well as the anonymous reviewers for their evaluations.

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