All insect-eating migratory birds who winter in Africa and breed in the Dutch woods have decreased in numbers since 1984. This has been revealed by research conducted by the University of Groningen, the SOVON Dutch Centre for Field Ornithology, Statistics Netherlands (CBS), Radboud University Nijmegen and Alterra, published on 16 December in the Proceedings of the Royal Society B: Biological Sciences.
This decline is dramatic for certain species: nightingales have declined by 37 percent, wood warblers by 73 percent and Ictarine warblers by 85 percent.
Due to climate change, spring is starting earlier and earlier in the year. On average, trees are in leaf two weeks earlier than 25 years ago, and the caterpillars who eat the young leaves are also appearing two weeks earlier. The eggs of many birds hatch at the moment when there are lots of caterpillars in the woods so that their young have enough to eat.
The researchers already knew that two of the woodland bird species have not been able to adapt their breeding periods sufficiently to the warming climate. Great tits and pied flycatchers now breed too late for the caterpillar peak. However, nothing certain was known about the exact consequences in terms of numbers for these and other bird species.
In Northern Europe, where spring has hardly shifted, the woodland birds are not declining in numbers. The resident birds in Dutch woods do not show a decline either. In addition, biologists do not see any decline in the Africa migrants who breed in our marshes.
Taking all of this together, the researchers conclude that the decline is not only the result of changing circumstances in Africa. In their view, the decline is mainly due to long-distance migrants not having adapted their migration sufficiently to the earlier appearance of the caterpillars.
Marsh birds that winter in Africa do not suffer from this. This is because lots of insects can be found in the marshes throughout the spring and summer. It's thus less important for these birds to breed at exactly the right moment.
Due to continuing climate change, it's probably going to get a lot quieter in the Dutch woods, fears Groningen biologist Dr. Christiaan Both: "Fewer and fewer migratory birds will be able to breed here."
The counts for the studies were mainly done by volunteer birdwatchers, who count breeding birds the same way every year in hundreds of different locations. It's thanks to their efforts that is has been possible to chart these consequences of climate change over the years.
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