You might guess that sometimes using field marks for identification is easy, but other times it's hard. For example, if you live in southern California and see a hummingbird, you'll need to figure out whether you're seeing a Calliope, an Anna's, a Black-chinned, a Costa's, a Rufous, or an Allen's, because they're all present, but all those species zip around so fast that it can be hard to see important field marks. Florida is home to about 17 species of sparrow, most of which are small, brownish birds with thick beaks; Texas has about two dozen duck species, many of them quite similar, and; in Michigan, you can see about 36 species of warbler, many of which are very similar.
On a recent trip to Mexico I sketched the following look-alike wrens. Can you figure out the distinguishing characters? The top two are especially hard. Notice that the Plain Wren has a pale lower back area (referred to as the rump among birders) while the Bar-vented Wren's rump area is dark.
If you want to be a good birder, you should develop general notions of what the various groups of birds in your area look like. You should learn to tell at a glance whether what you have is a falcon or a hawk, a duck or a goose, a warbler or a vireo. Only when you master the groups should you tackle the species. Your field guide will help you learn these groups. The Robbins field guide, Birds of North America, for instance, begins the Sparrows section with a reminder that "Sparrows are small, brown-bodied birds with streaked backs and short conical beaks," and then information about food and other matters follows.
When you're learning to distinguish bird groups, you must pay attention to gross anatomical and behavioral features. For example, one of the first birding groups most of us learn to identify is that of the woodpeckers. We know a bird is a woodpecker when we see that it has these three features::
Once you know your area's common groups of birds, then you must notice more subtle field marks. Does the thrush have a rusty rump (Hermit Thrush), or a rusty back and crown (Wood Thrush)? Is the cuckoo's beak yellowish (Yellow-billed Cuckoo) or black (Black-billed Cuckoo)?
Actually it's rare that just one field mark distinguishes a species from all others. Typically you must learn clusters of fieldmarks. For example, when identifying swallows you should focus first on how deeply the tail is notched, and whether the throat area is banded, dingy, reddish brown, or pure white. Let's say your swallow has a reddish-brown throat. North America is home to seven swallow species, but only two possess reddish brown throats. Of these two, the Barn Swallow's tail is very deeply forked, while the Cliff Swallow's tail is hardly forked at all.
Field guides always point out each species' most important field marks. For example, in Robbins's Birds of North America, the Orchard Oriole is described as " ...the only oriole east of the Mississippi River with a solid black tail." Therefore, if you live east of the Mississippi, just remember that a black tail, if you have an oriole, signifies the Orchard Oriole .