Kinglets

Birds in the Regulidae family are characterized by their tiny size, big rounded heads, small pointed wings, and small pointed beaks. They often have a patch on the top of their heads and are gray to olive in color. They eat insect and spider eggs and can eat insects and fruit while hovering in the air. There are only 6 species in this family, and only two found in North America. 
Included below is the Golden-crowned Kinglet and the Ruby-crowned Kinglet.

Golden-crowned Kinglet
Regulus satrapa

Habitat: Mostly conifers; in winter, sometimes other trees. 

Preferred Food: Mostly insects. Diet includes many eggs of insects and spiders. Will feed on oozing sap; rarely feeds on fruit. 

Fun Fact: Though barely larger than a hummingbird, this very active bird can survive -40 degree nights, sometimes huddling together for warmth.

Ruby-crowned Kinglet
Corthylio calendula

Habitat: Conifers in summer; other trees and brush in winter. 

Preferred Food: Mostly insects, also eats spiders and pseudoscorpions and eggs of insects and spiders. In winter, also eats some berries and seeds. Sometimes takes oozing sap or visits flowers, possibly for nectar.

Fun Fact: Metabolic studies suggest that Ruby-crowned Kinglets only use about 10 calories per day.