Resources

I try to provide references for every ID blog post I write. Here's a list of the sources I often use.

Websites

Identifying a bird when you have no clue where to start

• WhatBird - This step-by-step identifier lets you narrow down species based on the field marks you know. Specify a few Attributes within each Attribute Group and watch the list of matches narrow down! 
• Merlin - Actually a smartphone app. Give Merlin a bird photograph and a location, and it will attempt to identify the species. Good for figuring out what group of species to look at, but doesn't always get tricky IDs right.
• Facebook - If you're looking for a little more interactive help, you can post to one of the many excellent Facebook bird identification groups. Make sure to read the group rules before posting in any Facebook group.
• What's This Bird? will often give you an answer in minutes. Folks in that forum don't always describe what field marks led them to their identification; if you want to know this, make sure to ask. 
• There is also most likely a regional group for wherever you live.
• If you're lucky enough to live in Pennsylvania, our identification group is ID that PA Bird!, my personal favorite forum. Folks there are very friendly and will give you hints that help you learn how to identify your bird.
• Californians have Birding California, a more general birding group whose members are always happy to give ID help. Again, ask if you want to understand how someone made an identification.

Online species field guides

• Cornell's All About Birds - Contains descriptions of each bird, life history information, vocalizations, and lists of similar species. Amount of content for each species varies widely; information is often sparse on less-studied species. The "similar species" sections are often helpful, but sometimes miss obvious confusion species
• Audubon's Guide to North American Birds - This guide is written in a more brief style than All About Birds. However, it often includes a wider variety of vocalizations than AAB. If I'm identifying an unfamiliar species by sound, I always peruse similar species' vocalizations on Audubon before I make the final call. 

Other resources:

• Feather Atlas - Has images of flight feathers (the large wing and tail feathers) of many North American species.
• Macaulay Library - Here's one I visit a lot. A huge collection of wildlife images, videos, and vocalizations. If you want to get an idea of the natural variation in a certain species, look through different plumage stages, and hear a large sample of its vocalizations, look no further.
• Old Bird and Night Migrants - Two websites about the process of studying nocturnal flight calls (NFCs), the sounds birds make when they migrate at night. Ever wanted to hear hundreds of Swainson's Thrush calls in 10 minutes? Get into NFCs!
• Birds of North America - Comprehensive life histories of every bird species that breeds in North America. Sadly, it requires a paid subscription. A 30-day trial is only $5.00, though!

Field Guides

The Sibley's Field Guide to Birds of Eastern North America (2003 edition)

Sibley Birds West: Field Guide to Birds of Western North America (2016 edition)

Portable, easy-to-use guides. Some beginners may find these daunting, but they are generally useful to birders of all skill levels. Unfortunately, both the 2016 East and West editions have several typos and formatting errors (West errors, East errors). But they do have updated pictures and additional species compared to the 2003 versions.

The Warbler Guide by Tom Stephenson and Scott Whittle (also an app)

The Shorebird Guide by Michael O'Brien, Richard Crossley, and Kevin T. Karlson

These two guides are often heralded as the best guides for their respective species groups.

Identification Guide to North American Birds by Peter Pyle

Less of a field guide than an ornithologist's handbook. A highly technical guide meant to assist in the identification, aging, and sexing of birds in the hand.

Field Guide to Advanced Birding by Kenn Kaufman
A great book for learning the basics about bird structure and identification. It describes principles and pitfalls of identifying birds in the field. These principles are then applied in thorough discussions of some of the most common identification challenges in North American birding.