Saturday, October 30, 2010

Ageing hummingbirds

Most field guides mention rufous feather edging on the head and upperparts as a way to tell hatch year hummingbirds from older individuals in many species. Bird banders look for that, but they also look for something not visible in the field: bill striations. The bills of young hummingbirds are soft and often a little shorter (because still growing) than those of older birds. They also show little corrugations or striations - growth marks - along the edge of the upper mandible. Over time, as the hummingbird gets older, the bill gets harder and smoother, and these bill striations disappear.

Thus, the percentage of bill striations can be used to age the hummingbird in the hand. A sufficient amount of such data collected on a resident population of hummingbirds can give us clues about the timing of the breeding season, which in many hummingbird species is timed with the flowering of the species' favorite shrub or tree, and does not necessarily follow the breeding cycle of the majority of birds, which is usually timed around the rainy season in the tropics. Molt data provide additional clues, as most hummingbirds suspend their molt during the breeding season.

The bird at the top is an immature Rufous-tailed Hummingbird, the most common species in lowland areas in Costa Rica, and found from Mexico to western Venezuela. Clicking on the photo will open that photo in your browser window; clicking on it again will blow it up to fantastic detail. I suggest you do this to view the 80% bill striations score that this particular individual shows. A fine line of diagonal marks runs across the edge of the upper mandible. This individual probably hatched less than two months ago.

Here is another Rufous-tailed Hummingbird, caught at the same location (Tortuguero, Costa Rica) on the same day (October 28th, 2010). This bird shows only 40% of bill striations. From the base of the bill, until almost half of the bill, remaining striations or corrugations can be seen, but this bill is certainly more advanced in the process of smoothing and hardening than that of the previous individual. This bird is probably a hatch year bird also, but likely a few months older than the other bird.

Here's another species, a Bronzy Hermit. This is also a common species here in Tortuguero, one that I catch almost daily. Click on the photo, then click again, and determine for yourself what the bill striations score in this particular individual would be. I'll tell you what I think it is a little further below. If you accurately scored % bill striations, you're one little step advanced in Hummingbird Banding 101. (And if you're concerned that the bird looks a little stressed, with the tongue hanging out like that, I can tell you that it took off full throttle immediately after this photo was taken. I do have a bottle of sugar water ready to feed a tired hummingbird, but in this case it was not necessary to use it.)

This is a Long-billed Hermit. The rufous edging of the head feathers (and the rufous in the facial stripes) tells us this is another hatch year bird. About 60% of the bill shows striations.

OK, so what does an adult bill then look like? Here's an adult male Green-throated Mountain-gem, a highland species from Honduras, photographed September 2010. As you can see, his bill is smooth.

And finally an adult female of the same species, photographed during the same banding pulse, with less than 10% bill striations at the base of the bill. Some adults will retain up to 10% of bill striations.

The Bronzy Hermit (third photo from the top) shows about 40% bill striations, although the quality of the photo makes it hard to assess.

Postscript (nicely tying this and the previous post together):
The cadaver of the young hummingbird that the snake tried to eat a couple of days ago, shows something you'll never see on a hatch year temperate zone breeder: symmetrical wing molt. The bird was molting P9 in both wings. Symmetrical wing molt is a feature usually taken as indicative of an AHY (after hatch year, i.e. adult) bird. This hummingbird, however, also shows 50% bill striations, and therefore must be a HY bird. This is just one example of many where tropical birds defy our general notions of birds, largely based on temperate zone breeders. There still remains a lot to discover!

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