How to Know the Difference Between a Spring Warbler and a Fall Warbler

Difficulty: Moderate

Instructions

Things You'll Need:

Bird guide
Binoculars
An eye for detail

Step 1

Take Season Into Consideration

Very basic bird guides, to the extent that they include warblers at all, will usually picture and highlight the adult male of spring. This works fine of course for identifying adult males in spring, but not so much for adult males in fall! Some basic guides may include females (often less striking in appearance and may more closely resemble fall males than do spring males). Some may include the confusing fall plumages of young males. Most intermediate to advanced bird guides will include illustrations of all three.

Even if you do own a more comprehensive bird guide though, you must beware that usually adult males in spring are still the primary examples rendered. So what this all means is that if you're birding in spring, you don't have to be quite as detail-oriented about warblers as you do in the fall.
Step 2

Learn About Field Marks

Many birds, especially the commonly known ones, look the same year round, and in addition, may be obvious at first sight. Blue Jays, Black-capped Chickadees, House Sparrows, and even male Northern Cardinals look the same year-round and it doesn't take a discriminating eye to know them at first blush.

Some bird families however are difficult enough to know one species from another even in the full spring regalia of the male, much less when you throw seasons, gender, and age into the mix. So birdwatchers have to look for what are known as "field marks" in distinguishing one species from the next.
Step 3

See Examples of Field Marks

Let's take the basic species named in Step 2 for starters; they have field marks themselves even though we don't really need to concentrate on them. Blue Jays and Northern Cardinals of course have crests, tufts on top of their heads. Crests are a kind of field mark. Black-capped chickadees of course have caps, which are a kind of field mark. Generally speaking, field marks then are features of species that may be shared by other species (crests for instance), but that aren't shared by most, thus they aid in the process of elimination aspect of identification.

Besides caps and crests, more "esoteric" type fieldmarks include wing-bars (usually white stripes on wings - of which the number of them on each wing can come into play), eye-rings, tail spots (often flashes of white or color exposed as the tail fans), streaked or spotted versus plain breasts, colored legs (for instance pink as opposed to gray or black), "collars", and eye stripes or streaks. Field marks can also be behaviors such as tail-wagging, head-bobbing, undulating flight, "nervousness", etc. Songs and sounds function similarly to field marks in terms of identification, but they're not necessarily called field marks.
Step 4

With Warblers, It's All About Field Marks

Since most warblers give the same basic profile, small and often with yellow and/or greenish-olive someplace on them, even in spring sometimes you have to look at the breasts for streaks or the wings for wingbars. But in the fall, this sort of thing is paramount to aid in identification, since many of the more obviously distinguishing patches of color aren't there anymore to help.
Step 5

Take An Example

Please navigate to this page: http://www.allaboutbirds.org/guide/Chestnut-sided_Warbler/id

Look at the photo at the top of the page, "Chestnut-sided Warbler, male, breeding plumage". Compare it to the bottom photo, "Chestnut-sided Warbler, non-breeding plumage". Quite a difference! Hard to believe the two examples are the same species and sex (males and females similar in non-breeding plumage)!

Now locate the descriptions:

Male
Breeding (Alternate) plumage: Crown bright yellow, with white edge at front. Nape pale gray streaked with black. Back striped yellow and black. Tail blackish. Black stripe from bill to behind eye. Black mustache stripe. Cheek white. Throat, chest, belly, and under tail white. Sides with long narrow chestnut streak. Two broad yellowish wingbars. Flight feathers outlined in pale yellow or white.

Nonbreeding (Basic) plumage: Back, nape, and crown yellowish green. Some dark streaking on rump. Sides of head and neck pale gray. White eyering. Underparts dull white. Long thin streak of chestnut along sides. Two broad yellowish wingbars. Flight feathers outlined in pale yellow or white. Eyes dark. Legs dark.

Notice the painstaking description of field marks!

Lastly, read down just a bit further to where it says:

Similar Species

Bay-breasted Warbler also has chestnut on sides, but in breeding plumage face is dark and throat and crown are chestnut. In nonbreeding plumage Bay-breasted is dusky underneath and lacks the bright yellow-green of a chestnut-sided.
Step 6

Now Jump Out of the Nest and Fly!

This should give you just a taste of what it takes to identify birds in general, and especially warblers. You might want to play around on the site featured in Step 5, comparing different species of warbler. Toward the bottom of this page, under Resources, I've provided a direct link (first link) to the different kinds of warblers you can compare and contrast. The second link is a related eHow article.