the warbling vireo (vireo gilvus) is a small north american songbird .
its breeding habitat is open deciduous and mixed woods from alaska to mexico and the florida panhandle .
it often nests along streams .
it migrates to mexico and central america .
adults are 12 cm (4.7 in) long and weigh 12 g (0.42 oz) .
they are mainly olive-grey on the head and upperparts with white underparts; they have brown eyes and the front of the face is light .
there is a white supercilium .
they have thick blue-grey legs and a stout bill .
western birds are generally smaller and have darker grey crowns .
warbling vireos forage for insects in trees , hopping along branches and sometimes hovering .
they also eat berries , especially before migration and in winter quarters , where they are  like other vireos  apparently quite fond of gumbo-limbo seeds , though they will not venture into human-modified habitat to get them .
they make a deep cup nest suspended from a tree branch or shrub , placed relatively high in the east and lower in the west .
the male helps with incubation and may sing from the nest .
the warbling vireo's song is a cheerful warble , similar to that of the painted bunting .
there are subtle differences in song between eastern and western birds , at least where the ranges meet in alberta .
some authorities split the eastern and western races of this species into separate species: the western warbling vireo , v .
swainsoni , includes v .
g .
swainsoni , which breeds from southeastern alaska and southwestern northwest territories to the sierra san pedro mrtir , baja california , and v .
g .
brewsteri , which breeds from southern idaho , wyoming , and montana to south-central oaxaca .
these two subspecies winter in mexico .
the swainsoni group also includes v .
g .
victoriae , an isolated population breeding in the sierra de la laguna , baja california sur , and migrating to unknown wintering grounds .
the eastern warbling vireo , v .
gilvus , breeds from central alberta and northern montana east and south through most of the united states and parts of southern canada , outside the range of the previous group .
it winters south of the isthmus of tehuantepec from south-central chiapas to nicaragua .
it completes its autumn molt on the breeding grounds , while the swainsonii group completes it after leaving .
the brown-capped vireo (vireo leucophrys), resident in central america and northern south america , is sometimes considered conspecific with the warbling vireo .
