Baby season at the bird banding station

The birds are having babies, those babies are learning to fly, and they are flying into our nets at the banding station. They’re not really babies by this point: most of them are independent of their parents. They may have been out of the nest for a month or more, and are technically “juveniles” or “immatures.”

At the banding station we collect data on each bird we catch, including that bird’s age. It’s important to record the age if we can, because the more accurate we are with the age the first time we catch a bird, the more accurate we can be later. If we caught a bird in 2014 and recorded that it was a juvenile, then when we catch it in 2016, we’ll know it’s exactly 2 years old. If we didn’t bother to age it back in 2014, then in 2016 we would only know that it was at least 2. That maybe seems like a small distinction, but the lifespans of wild birds are still an area in which we lack a lot of information, so knowing exact ages is valuable.

How do you tell if a bird is a juvenile or an adult? In some species, the juveniles are dramatically different colors than the adults. Juvenile juncos, brown and streaky, look distinct from adults even from a distance—until they molt, at least. But the differences can be a lot more subtle.

Here is a Common Yellowthroat from the banding station:

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How old is it?

And no—no points for knowing that it isn’t an adult male.

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Adult male Common Yellowthroat

Is it a female or a juvenile? Sometimes we know birds are adult females because they have brood patches, a physical trait—lost feathers and spongy skin on the abdomen—that develops to allow females to incubate their eggs. (In some species males have them too, though.) But you have to be careful even with that, because you can confuse the brood patch with “baby belly,” the lack of feathers that juveniles have on their abdomen.

This is an adult female Common Yellowthroat:

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The bird in the first picture is a juvenile. You can tell this from the picture in three ways:

  1. Yellow gape at the corner of the mouth.
  2. Overall buffy (yellow-brown) color of the breast.
  3. Looser, less shape-holding downy feathers (it’s why the bird looks so mussed up).

The yellow gape is the easiest to see. Here it is again:

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The buffy color is harder to distinguish, but if you compare the photo below (another juvenile) to the photo of the female, you may be able to see it. The juvenile is buffy, while the female is a cleaner yellow.

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The first time I saw a juvenile Common Yellowthroat at the banding station, it looked so different in coloration from a female Common Yellowthroat that I got all excited, convinced that it was some new migrant species. “It’s all peachy-colored!” I told everyone. I still think the “buff” looks kind of peachy, but apparently that isn’t the general opinion.

There’s even a fourth trick with Common Yellowthroats, although you couldn’t use it in the photo I’d given you: juveniles have buffy wing bars, while the adults don’t.

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Buffy wing bars in the juvenile.

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This juvenile is in the process of molting those feathers.

Bewick’s Wrens don’t have any dramatic coloration differences between age classes. Here are two Bewick’s Wrens; which is the juvenile?

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Bewick’s Wren #1

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Bewick’s Wren #2

Wren #2 is the juvenile: it has that yellow gape. (It also has a slightly narrower superciliary stripe, and the stripe is slightly more buffy.)

Now let’s try the Song Sparrow. Which of these birds is the juvenile?

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The yellow gape doesn’t really help in this case, since both seem to have it a tiny bit. The gape shrinks as the bird gets older, so if you have an older juvenile, it won’t help you. Again, the key here is going to be a buffy color: the bird on the left, with the yellower face, is the juvenile.

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Another juvenile Song Sparrow: buffy wash, less distinct facial markings.

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Adult Song Sparrow

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Adults also tend to be angrier at us than juveniles, but we don’t use that as an ageing criterion.

We can compare the Song Sparrows to see the different textures in the downy feathers too. Compare the undertail coverts (the feathers underneath the tail):

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The juvenile’s feathers (top) are less dense, less coherent. The bird books call them “loosely textured.” The adult’s undertail coverts are more shapely, more dense, and even have an elegant little color pattern.

Something like the texture of the downy feathers is not something that you’ll likely be able to see through binoculars. These techniques are very specific to situations where you have the bird in hand. And they are species-specific: those buffy wing bars only help you in Common Yellowthroats. At the banding station we have a large collection of reference materials, including the indispensable Pyle Guide, to help up keep track of what to look for in each species.

Sometimes you have a species with no color differences at all, and you can’t always rely on the yellow gape (it might be an older juvenile) or on the texture of the feathers (it can be hard to tell unless you have an adult right there for comparison). One such species is the Bushtit.

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Bushtit (holding a toothpick, because he kept grabbing onto the net, so we gave him something else to grab).

For Bushtits, we rely on “skulling.” We skull all our birds, in fact, but in birds like Bushtits, skulling is our main clue to age. Skulling is the act of looking at the bird’s skull (fortunately, birds have quite translucent skin; we simply peer between the head feathers and look at the skull through the skin) and judging how developed it is based on the color. As the bird gets older, the skull changes from being a single layer of bone to being two layers separated by air but connected by tiny bone columns. The double-layer bone is a lighter color than the single-layer bone. The larger the area of the skull that is the light color, the older the bird is.

Skulling is not easy—it takes a lot of practice, and I’m still learning—but it is fascinating. You can see the bone growth in a living animal without harming it at all. I wish more animals were transparent!

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Juvenile Bushtit. (Also, female: Bushtits with yellow eyes are female.)

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2 thoughts on “Baby season at the bird banding station

  1. Are you working on a MAPS banding project as well? I’m doing that up in northern Alberta. We just started our first banding day today.

    Do you have both lincoln’s and swamp sparrows in your area? Any advice on how to tell the juveniles apart?

    • As far as I know, this banding station isn’t MAPS-affiliated. I just volunteer there about once per week. (I’d love to be there more, but I also need to do things that make money, unfortunately…)

      We have Lincoln’s, but they don’t breed here; I haven’t seen any since the winter. No Swamp Sparrows. Does Pyle not talk about the juveniles?

      I wish I could band all over the continent – it would be so neat to see all the birds we don’t get. We get a pretty small slice even of the birds that *are* local to the SF Bay Area, just because of the habitat we’re in and the placement of the nets.

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