Monday, 19 July 2010

Willow Warblers struggle for success

It has been a tough few weeks for some of the Willow Warblers nesting on the local nature reserve. These diminutive birds are summer visitors to our shores, familiar to most birdwatchers for their soft descending verse and rather plain plumage. Breeding across a range of scrubby and woodland habitats, they are one of the northern summer’s commonest breeding birds. The delicate appearance of this ‘leaf’ warbler (as this and other closely related species are collectively known) is matched by a delicately formed nest, placed at ground level and usually well hidden from prying eyes.

Finding Willow Warbler nests can be tricky, especially early in the season if you have missed them actively building their domed construction of moss, grass and bracken. We first locate territory holding males and then spend time watching to see if we can locate the female, who sometimes gives away her presence when off the nest by a characteristic call note. If this fails to reveal the female or her nest then we will very carefully search through those areas which seem to hold suitable ground cover for nest placement. Because the nest is so well camouflaged, every step has to be considered and checked before a footfall is placed; each tuft of grass or tangle of bracken has to be gently tapped and then inspected. It is slow and steady work but important nonetheless, as we strive to collect information on breeding success (such as clutch size, brood size and number of chicks fledged) for the BTO’s Nest Record Scheme. In a typical year fewer than 150 Willow Warbler nests are found and visited nationwide, yet still a sufficient sample for the BTO to produce national figures in support of important conservation work.

Some of our birds have struggled this year, though quite possibly no more so than in a typical year, with several falling victim to nest predators like crows, dogs and, we rather suspect, snakes. Some of the nests have been located close to old gravel pits, in areas where we often encounter basking Grass Snakes and these reptiles could easily polish off a nest of young warblers. When we find a predated nest we look to see if and how the nest itself has been damaged. Because Willow Warblers make a domed construction, with a small entrance hole in the side, most predators will inflict a certain amount of damage as they try to extract the nestlings or eggs from within. Nests that appear undamaged may have fallen victim to snakes or Weasels, both of which are small enough to use the entrance hole provided by the birds. It would certainly be interesting to deploy cameras to monitor some of these nests next year.

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