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201027Dec16:36

Decrease of migra­tory birds in Dutch forests

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pub­lished 27 Decem­ber 2010 | mod­i­fied 27 Decem­ber 2009
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All insect-​eating migra­tory birds who win­ter in Africa and breed in the Dutch woods have decreased in num­bers since 1984, accord­ing to Both et al. 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. But when birds do adapt their breed­ing peri­ods suf­fi­ciently to this change in cater­pil­lar peak it can have dra­matic con­se­quences in terms of num­bers for these 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. (Source: www​.sci​encedaily​.com)

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