Ilkley Eastside Wander #17: Spot the Chiffchaff?
This is the 17th of a series of 20 caches which takes you through some of the lovely countryside immediately south-east of Ilkley. The cache a small, screw-capped brown pot, is hidden at a gate on the edge of a copse.
As I was placing the cache, this commonly heard but difficult to spot little bird was calling strongly from the patch of woodland beyond the gate. This dainty warbler is named after its song and is increasingly taking advantage of the UK’s warming climate by staying here all year long.
See GC8PCW2 Ilkley East Wander #1: Intro and . . . Prick!? for information on the series and waypoints. See Gallery for an annotated map with cache locations and parking spots.
If doing Full Series in sequence: from #16, continue south-east along the path until you pass through a gate onto a lane. Turn left (north) and follow this to the cache location.
If doing Mini-Series 2: park at P3 and refer to IEW#1 using the link above to reach IEW#13 then continue with the shorter sequence to IEW#16 then as above.
The (common) chiffchaff (Phylloscopus collybita) is a common and widespread leaf warbler which breeds in open woodlands throughout northern and temperate Europe and the Palearctic. It is a normally a migratory passerine which winters in southern and western Europe, southern Asia and north Africa. Greenish-brown above and off-white below.
The female builds a domed nest on or near the ground, and assumes most of the responsibility for brooding and feeding the chicks, whilst the male has little involvement in nesting, but defends his territory against rivals, and attacks potential predators.
A small insectivorous bird, it is subject to predation by mammals, such as cats and mustelids (eg. badger, stoat) and birds, particularly hawks of the genus Accipiter such as the sparrowhawk. Its large range and population mean that its status is secure.
In 1789, the British naturalist Gilbert White was one of the first people to separate the similar-looking common chiffchaff, willow warbler and wood warbler by their songs. See here for ID guide.
There are three commonly accepted subspecies of which the one occurring here is the nominate P. c. 
collybita which breeds in Europe east to Poland and Bulgaria. It mainly winters in the south of its breeding range around the Mediterranean and in North Africa but has been expanding its range northwards into Scandinavia since 1970.
Its English name is onomatopoeic, referring to its chiff-chaff song (listen/see here and here which is one of the first avian signs that spring has returned. Its call is a hweet (listen here).
It is a small, dumpy, 10–12 cm long leaf warbler. The male weighs 7–8g and the female 6–7 g. The spring adult has brown-washed dull green upperparts, off-white underparts becoming yellowish on the flanks, and a short whitish supercilium. It has dark legs, a fine dark bill, and short primary projection (extension of the flight feathers beyond the folded wing).
As the plumage wears, it gets duller and browner, and the yellow on the flanks tends to be lost, but after the breeding season there is a prolonged complete moult before migration.
The newly fledged juvenile is browner above than the adult, with yellow-white underparts, but moults about 10 weeks after acquiring its first plumage. After moulting, both the adult and the juvenile have brighter and greener upperparts and a paler supercilium.
When not singing, it can be difficult to distinguish from other leaf warblers with greenish upperparts and whitish underparts, particularly the willow warbler. However, that species has a longer primary projection, a sleeker, brighter appearance and generally pale legs. It also has rounded wings in flight, and a diagnostic tail movement consisting of a dip, then sidewards wag, that distinguishes it from other Phylloscopus warblers.
It breeds across Europe and Asia east to eastern Siberia and north to about 70°N, with isolated populations in northwest Africa, northern and western Turkey and northwestern Iran. It is migratory, but it is one of the first passerine birds to return to its breeding areas in the spring and among the last to leave in late autumn.
When breeding, it is a bird of open woodlands with some taller trees and ground cover for nesting purposes. These trees are typically at least 5m high, with undergrowth that is an open, poor to medium mix of grasses, bracken, nettles or similar plants. Its breeding habitat is quite specific, and even near relatives do not share it; for example, the willow warbler (P. trochilus) prefers younger trees, while the wood warbler (P. sibilatrix) prefers less undergrowth.
In winter, it uses a wider range of habitats including scrub, and is not so dependent on trees. It is often 
found near water, unlike the willow warbler which tolerates drier habitats.
It is highly territorial during the breeding season, with a core territory typically 20m across, which is fiercely defended against other males. Other small birds may also be attacked. The male is inquisitive and fearless, attacking even dangerous predators like the stoat if they approach the nest, as well as egg-thieves like the Eurasian jay. His song, given from a favoured prominent vantage point, appears to be used to advertise an established territory and contact the female, rather than as a paternity guard strategy.
Beyond the core territory, there is a larger feeding range which is variable in size, but typically 10+ times the area of the breeding territory. It is believed that the female has a larger feeding range than the male. After breeding has finished, it abandons its territory, and may join small flocks including other warblers prior to migration.
The male returns to its breeding territory 2-3 weeks before the female and immediately starts singing to establish ownership and attract a female. When a female is located, the male will use a slow butterfly-like flight as part of the courtship ritual, but once a pair-bond has been established, other females will be driven from the territory.
The male has little involvement in the nesting process other than defending the territory. The nest is usually 12.5cm high and 11cm across and built on or near the ground in a concealed site in brambles, nettles or other dense low vegetation. The domed nest has a side entrance, and is constructed from coarse plant material such as dead leaves and grass, with finer material used on the interior before the addition of a lining of feathers.
The clutch is 2-7 (usually 5-6) cream-coloured eggs which have tiny ruddy, purple or blackish spots and are about 1.5 cm long and 1.2 cm across. They are incubated by the female for 13–14 days before hatching as naked, blind altricial chicks.
The female broods and feeds the chicks for another 14–15 days until they fledge. The male rarely participates in feeding, although this sometimes occurs, especially when bad weather limits insect supplies or if the female disappears. After fledging, the young stay in the vicinity of the nest for 3-4 weeks, and are fed by and roost with the female, although these interactions reduce after approximately the first 14 days. In the north of the range there is only time to raise one brood, due to the short summer, but a second brood is common in central and southern areas.
Although pairs stay together during the breeding season and polygamy is uncommon, even if the male and female return to the same site in the following year there is no apparent recognition or fidelity.
Like most Old World warblers, it is insectivorous, moving restlessly though foliage or briefly hovering. It has been recorded as taking insects, mainly flies, from more than 50 families, along with other small and medium-sized invertebrates. It will take the eggs and larvae of butterflies and moths, particularly those of the winter moth. It has been estimated to require about 1/3 of its weight in insects daily, and feeds almost continuously in the autumn to put on extra fat as fuel for the long migration flight.
As with most small birds, mortality in the first year of life is high, but adults aged 3-4 years are common with a record of more than 7 years. Eggs, chicks and fledglings of this ground-nesting species are taken by stoats, weasels and crows such as the European magpie, and the adults are hunted by birds of prey, particularly the sparrowhawk. Small birds are also at the mercy of the weather, particularly when migrating, but also on the breeding and wintering grounds.
It is occasionally a host of brood parasitic cuckoos, including the common cuckoo, but recognises and 
rejects non-mimetic eggs and is therefore only rarely successfully brood-parasitised. Like other passerine birds, it can also suufer from intestinal nematode parasites and external ticks.
The main effect of humans on this species is indirect, through woodland clearance which affects the habitat, predation by cats, and collisions with windows, buildings and cars.
It has an enormous range, with an estimated global extent of 10 M sq.km and a population of 60–120 million in Europe alone, so its conservation status is - not surprisingly - 'of least concern'.
See here for comprehensive info and some fine photos.