This tiny bird is a Pine Siskin (Carduelis pinus). Flocks of siskins may monopolize your thistle feeder one winter and be absent the next. This nomadic finch ranges widely and erratically across North America each winter in response to seed crops. Better suited to clinging to branch tips than to hopping along the ground, these brown-streaked acrobats flash yellow wing markings as they flutter while feeding or as they explode into flight. Flocks are gregarious, and you may hear their insistent wheezy twitters before you see them.
They often visit feeders in winter or cling to branch tips of pines and other conifers, sometimes hanging upside down to pick at seeds below them. They are gregarious, foraging in tight flocks and twittering incessantly to each other, even during their undulating flight.
Although Pine Siskins prefer coniferous or mixed coniferous and deciduous forests with open canopies, they are opportunistic and adaptable in their search for seeds. They’ll forage in weedy fields, scrubby thickets, or backyards and gardens. And they’ll flock around feeders, especially thistle feeders, in woodlands and suburbs.
Text from http://www.allaboutbirds.org