the chestnut-sided warbler (dendroica pensylvanica) is a new world warbler .
they breed in eastern north america and in southern canada westwards to the canadian prairies .
they also breed in the great lakes region and in the eastern usa .
these birds are migratory , wintering in central america south to northern colombia , with an unconfirmed sighting from as far south as ecuador; they are also very rare vagrants to western europe .
they arrive in their breeding range in may and depart by mid-september .
this species is a moderately-sized new world warbler .
despite having very different plumage , it is thought to be closely related to the widespread yellow warbler .
in total , this species measures from 10 to 14 cm (3.9 to 5.5 in) in length and spans 16 to 21 cm (6.3 to 8.3 in) across the wings .
body weight ranges from 8 to 13.1 g (0.28 to 0.46 oz) .
among standard measurements , the wing chord is 5.7 to 6.8 cm (2.2 to 2.7 in), the tail is 4.2 to 5.8 cm (1.7 to 2.3 in), the bill is 0.9 to 1 cm (0.35 to 0.39 in) and the tarsus is 1.7 to 1.9 cm (0.67 to 0.75 in) .
in the summer , male chestnut-sided warblers are unmistakable in appearance .
they display dark-streaked gray backs , white faces , black eyestripes and greenish crowns .
their underparts are white , with chestnut flanks , and they also have two white wing bars .
the adult females resemble washed-out versions of the summer male , and in particular , the females lack the strong head pattern , and also have little to no chestnut coloring on their flanks .
non-breeding birds of both sexes have greenish heads , and greenish upperparts which are usually unstreaked .
they also have unstreaked pale grey breasts .
their wing bars are always present in their plumages .
their lack of streaking and greenish backs helps to distinguish this species from the larger blackpoll warbler in the fall .
the songs are high whistled lines often described as pleased , pleased , pleased to meecha .
this accented song is used primarily to attract a female and decrease in frequency once nesting is well under way .
males also sing unaccented songs (without the meecha at the end) and these are used mostly in territory defense and aggressive encounters with other males .
some males sing only unaccented songs , and they are less successful at securing mates than males that sing both songs .
their calls are harsh chips .
the chestnut-sided warbler has benefited from the clearing of mature forests .
they make use of the abundant second growth habitats .
in the tropics where they winter however , the species occurs mostly in mature tropical rainforests .
their cup-shaped nests are placed in a low bush , which is usually located in young deciduous woodland or scrub .
these birds lay 35 eggs that are creamy white or greenish with brown speckles in color .
the nest is a small cup woven of bark strips , weed stems , grasses , and plant down .
the nest is usually placed in a small crotch of a shrub or vertical tangle of vines no more than 2 m (6.6 ft) above the ground .
this species is frequently parasitized by brown-headed cowbirds .
this bird's numbers have increased as second growth forest became more common in the east in the late 19th century; their numbers have declined slightly since then .
chestnut-sided warblers are insectivorous , but will include berries in their winter diets .
they forage actively in shrubs and small trees , and sometimes will attempt to catch insects in mid-air .
