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Willow Warbler (Phylloscopus trochilus)

The soft descending scale of the Willow Warbler is one of Europe’s dominant sounds of spring. Everywhere from the fringes of the Mediterranean to deep into the Arctic Circle this small, sprightly foliage-gleaning warbler arrives from Africa to become a common bird of scrub and low trees. It is much associated with transitional vegetation, be it new growth appearing in previously cleared areas, or the last substantial vegetation at the tree-lines in mountains and tundra. The Willow Warbler is commonest in the north and is the most numerous bird in Scandinavia, with at least 24 million breeding pairs. It is possible, indeed, that no other summer visitor from Africa outnumbers it in total population. It wasn’t always so, of course. The activities of people opening up Europe’s vast forests have no doubted benefited this pioneering edge-habitat species.

The Willow Warbler is the epitome of the Phylloscopus or “Leaf” Warblers, a tit-sized plain yellowish bird that forages actively among the foliage (Phylloscopus means “examiner of leaves”) well above ground. Were it not so fidgety in its movements it would be a difficult bird to detect among the backdrop of a million leaves. It feeds from the surface of these, plus twigs and branches, principally taking soft-bodied meals such as aphids and caterpillars, and besides gleaning it will also make short aerial sallies to catch flying insects.

Male Willow Warblers arrive about 10 days before the females and immediately set up territories by song. These territories are important, since they will provide everything a bird, its mate and young may need, including a nest-site and all of their food. Not surprisingly, competition is strong, and Willow Warblers can become extremely aggressive in defence of their patch. It appears that prior residence is an advantage because, when a male returns to its territory of last year, only to find it occupied by an early arriving interloper, it always seems to win its precious land back.

Willow Warbler

Willow Warbler, Wiltshire, UK (Dave Kjaer)


The territories are no less important to the females, of course, and the fairer sex are highly judicious in their choosing of a potential site and its owner. It seems that each will visit several potential mates before it pairs up, spending 10-20 minutes in each territory and monitoring the incumbent’s song output. It will pay particular attention to how often the male sings and how much it forages. If a bird has a high song output and does not forage much, this implies that it finds food easily in its territory and does not have to search very hard. A bird in a lower quality territory will spend more of its time foraging, and will consequently have a lower song output. Thus, simply by listening in, a female can make value judgments about a male and, in particular, about the patch of ground where she might potentially hunt for food for her young.

This latter aspect is especially important because, as in the Chiffchaff, the female Willow Warbler often finds itself in the role of main provider for the family. Male Willow Warblers are probably not as indifferent to mates and family as Chiffchaffs, but they are more regularly polygynous, with some males taking two, and others three different mates, and such birds cannot provide for them all. Thus the burden of feeding is often taken largely or completely by the females.

The hard-pressed female also builds the nest, although the male often accompanies her when she is doing so and provides moral support. As is the case with other Phylloscopus warblers the Willow Warbler’s nest is a domed structure made out of grass and leaves, and whilst that of the closely related Chiffchaff is usually constructed above ground, the Willow Warbler’s is placed upon it.