Adult Chestnut-backed Chickadee, left, and recent fledgling, right; probably parent and offspring, since they were caught together. You can see the difference between feathers grown a year ago, in the adult, and feathers grown just a few weeks ago. Feathers really do wear down and fade over time.
To identify the species, sex, and age of a bird, a bird bander in North America relies either on personal experience with the species or on the massive handbook of bird descriptions known as the Pyle Guide. The Pyle Guide is full of confidence-instilling descriptions like “Juvenile rectrices usually pointed, but occasionally truncate” and “Male scapulars brownish-black, compared to blackish-brown in female.” To make matters still more confusing, birds can vary from site to site, meaning that the description in Pyle based on a population 100 miles distant may not be correct for your own local population. One of the best tools a bird bander can have, as they squint at a bird and wonder whether its iris is “red-brown” or “maroon” and whether that is even relevant to their local birds, is a comparison photo showing local birds.
You have to be a bit lucky to capture such photos: you have to happen to catch both birds at the same time. When you usually catch five or fewer birds at a time, as we do, the chances that any two of them will be an informative comparison becomes small. Each of these pair photos is a little special for this reason.
Juvenile Wilson’s Warblers: top, female; bottom, male. These are from a location with especially black crowns, so general tips for sexing the species based on crown color don’t always work. For example, in many other populations the top bird would be identified as male based on her having any crown black at all. Data from this specific population, including photos and measurements, help illuminate these differences.Female Red-winged Blackbirds: left, a one-year-old; right, a bird two years old or older. The older bird has some red in her epaulet.Individually, these two species look remarkably similar. Together, the Hutton’s Vireo (top) is obviously larger, with a sturdier bill, than the Ruby-crowned Kinglet (bottom). Not pictured: the angry call of a Hutton’s Vireo that clues you in that you may not be dealing with a meek kinglet.Family photo: both Bullock’s Orioles (left) and Red-winged Blackbirds (right) belong to the avian family Icteridae, which is generally distinguished by glossy black, touches of bright red/orange/yellow, and that hefty pointed bill.Photos aren’t perfect. This one was supposed to show a stronger yellow wash on one of these Warbling Vireos, but it’s quite hard to see. Light conditions, camera quality, the birds’ refusal to pose in the same way – all of these can make comparison photos less useful.Downy Woodpecker adult (left) and juvenile (right).The adult male has much more red and in a different pattern than the juvenile. You may also be able to see that the texture of their head feathers looks different: young birds generally have a more fluffy appearance, caused by what is technically called “loosely-textured” plumage. Presumably this is related to chicks’ need to grow a whole birds-worth of feathers in a short amount of time—fluffier, lower-quality feathers would be easier to grow.Two very similar-looking species that are important to distinguish: Pacific-slope Flycatcher, with an almond-shaped pale eyering, and Willow Flycatcher, with no eyering. Willow Flycatchers are of conservation concern.Spotted Towhees’ red eyes, besides making them look hysterical in every photo, can help you determine their age and sex: older birds and males have brighter, redder eyes. The bird on the left here has a brighter eye compared to the more dull orange eye of the bird on the right.A comparison of the face of an adult Song Sparrow, left, and a juvenile, right. The juvenile has those fluffy baby feathers, as well as an overall yellower color. (This way of holding birds looks odd but it’s safe for the bird.) The shoes in the photo are those of banders gathered around to see the comparison.
There are many other comparison photos I would like to have for reference and training. I’ll have to hope I get lucky.
I can see how experience would help, too: over the years, one would learn the tricks of light. Do you print out these photos and keep them for reference?
Tough Little Birds 
I think your title’s wrong. I think it should be “Two Birds in the Hand is Worth 10,000 Words”!
I could be accused of being a bit verbose XD
No matter! The photos are wonderful.
I really enjoyed this post. The photos were excellent in showing comparisons… something we rarely see. Nicely done!!!
I can see how experience would help, too: over the years, one would learn the tricks of light. Do you print out these photos and keep them for reference?
Yep! We do print out the best ones and keep them in our banding binders for reference. We also use them in online trainings.