Answer by Neil Kelley, paleontologist, geologist, naturalist, writer, educator
Step 1: Figure out what to expect in your area.
Among the several raven, crow, grackle, and blackbird species that occur in North America, only a handful typically occur in a given region. Take a look at range maps in a field guide or an online bird guide (such as this one from the Cornell lab of Ornithology:) to get an idea for what species are likely to occur in your backyard. Regional/local bird guides are available for many regions. You'll also want to consider cowbirds, magpies, starlings, and if you are in southern Texas or Florida, anis.
Note that "blackbird" refers to two very different types of bird in Europe and North America (although both are small to medium sized blackbirds). This just underscores how critical it is to become acquainted with your regional fauna before you start trolling through google image search looking for a match. The rest of this answer is written from an explicitly North American perspective, but the general tips should apply anywhere, once you have familiarized yourself with what species are likely to occur where you are.
Step 2: Consider size.
Crows and ravens are notably large for songbirds, larger than pigeons. Blackbirds and grackles are smaller than pigeons, as are starlings and cowbirds. With grackles being a bit larger than the others.
Elsewhere in the world, there are smaller black corvids (crow family) like jackdaws, but here in North America, it is pretty easy to distinguish the large black corvids (ravens and crows) from the smaller black icterids (blackbirds, grackles and cowbirds) based on size alone.
Step 3: Next consider features like beak, tail, plumage.
Bill shape and tail shape provide the key features to distinguish ravens and crows. Long tails characterize grackles and specific size and shape helps to differentiate between our three grackle species. Blackbirds have moderate-sized beaks and tails but different species have distinguishing marks in the plumage (yellow heads, red wings, glossy sheen etc.) Cowbirds have thicker beaks and shorter tails. Starlings have very short tails and pointy beaks (which are bright yellow in the breeding season unlike any of the others).
These differences might seem overwhelming, but if you refer back to step 1, you might have a list of four-six species that are likely to occur in your region, which makes identification easier. Generally speaking, it helps to learn the characteristics of larger groups (corvids vs. icterids, grackles vs. blackbirds vs. cowbirds), which will then guide you toward the members of those groups that are likely to be seen in your area.
Step 4: Acquaint yourself with behavior and vocal clues to seal the deal.
This is getting into serious bird nerd territory, but once you spend some time observing these birds, you will notice that they differ in characteristics like movement, foraging style, and sound. While many guidebooks provide some description of these characteristics, it really takes a bit of dedicated observation to become familiar with these clues.
For example, crows tend to be noisy, suspicious, especially aggressive toward raptors and owls, and hop around along the ground while they forage. Ravens have a very distinctive ("crock" vs. "caw") call and are often observed in pairs (crows frequently aggregate in larger groups).
Once you have spent a little time observing the bird species in your backyard, these subtle differences in behavior and call are immensely informative in making an ID even when you are unable to get a good clear look at an individual bird.More questions on Birds: