Instructions
Define the bird's size. A baby great blue heron is substantially larger than a baby meadowlark, both of which have long beaks in proportion to their size. A baby hummingbird will be at the extreme end of small -- they may be an inch long and weigh less than a dime.
Consider the shape of the beak. Many North American birds with long beaks also have relatively straight ones; again, the hummingbird demonstrates a convenient extreme, with its proportionately enormous bill forming a rigid, severe line. Wood storks, ibises and some other wading birds, however, have gently curved bills. The long beaks of many seabirds, such as gulls and petrels, have a sharp curve at the end.
Examine the bird's major physical features beyond its beak. A baby heron or egret can quickly set itself apart from other nestlings or fledglings by its long, gangly legs and long-toed feet. A baby kingfisher has a pink mouth and partly fused toes.
Look at the plumage pattern if the baby bird has feathered out. This may be less illustrative than you might think, considering that young birds typically develop downy feathers first and may not develop a characteristic adult pattern and color until nearly mature. Young gulls and terns often show dark speckling on their downy coats. A young woodpecker that has grown stiffer feathers will show the black-and-white plumage so distinctive in the adults of many species, such as downy and three-toed woodpeckers.
Think about the location in which you find the baby bird. A downy fledgling encountered on seashore rocks is likely a seabird -- maybe a gull, tern, cormorant or the like -- or a surf-foraging species such as a sandpiper. One encountered in marsh reeds is most likely a heron or egret. A long-billed baby in a backyard, thicket bramble or woodland groundcover may well be that of a passerine, or songbird.