Presque Isle State Park Bird Banding

Molt Indigo

Much of what I show on this blog relates to identifying, aging and sexing different bird species. One of the best ways to appreciate bird molts is to illustrate the process with a distinctive species.

Can you get more distinctive than this?

It takes two years for male Indigo Buntings to achieve this, their full adult plumage. If you look at the wings, you can see that the primary coverts (feathers just below the alula on the outer edge of the wing) are edged with blue. The greater coverts (immediately to the right of the alula) have broad blue edges. Even the flight feathers themselves show some bluish edging. This is because the adult birds replace all of their feathers after breeding, in what is termed their pre-basic molt (as an aside, molt terms can be rather confusing, a good review by Steve N.G. Howell is here). Depending upon the species, some birds will then replace many if not most of their body feathers in late winter in preparation for the breeding season. This is called their pre-alternate molt. There are many variations on this theme,

Indigo Buntings having one of the more complicated molt schedules. Juveniles fledge, then undergo a pre-supplemental molt where they replace most if not all of their body plumage. They then migrate south and undergo a complete pre-basic molt on the wintering grounds, replacing all their feathers. Finally, they undergo their first pre-alternate molt before heading back to the breeding grounds (or once they get there), replacing their secondary coverts, several inner secondaries and up to four tail feathers (more details here). During each of these molts, an individual bird may retain some of their older feathers, so by the time they return to the breeding grounds an SY bird may have retained juvenal and basic feathers, as well as the newly acquired alternate (breeding) plumage feathers.

This second-year (SY) male shows three different-aged feathers. His pale belly comes from retained juvenal feathers that weren't replaced during the pre-supplemental molt. The brown body feathers actually have blue bases, but the brown tips haven't worn off yet and block his true blueness. He has new secondary coverts, but some retained ones from his first pre-basic molt that are paler and more worn than the newer ones. All in all, he's a complicated individual.

Here's a nice view of his back. showing the difference in age and wear of his flight feathers. You can see that he's retained a few of his tail feathers but replaced several with darker, blue edged ones. He's kept some of his greater coverts, but replaced all of the median coverts (row of smaller feathers above the greater coverts)

So that's what I spend much of my time doing, trying to decide how old these birds are based on the shape, color and wear of their feathers. It's a lot of fun and similar to solving a puzzle, except that no one publishes the correct answers online the following day.

Here's a female Indigo Bunting, that I aged as an SY based upon different-aged coverts and replaced flight feathers.

Finally, this male added some confusion. I have no idea what went wrong with his molt, but I'm guessing some form of leucism?

That's all for tonight, more tomorrow.

P.S. Today (28 May) we netted another SY male, and since I had volunteers with me, one of them held its wing out so I could get this picture. Let's take a closer look at confusing bunting molt.

He retained his juvenal primary coverts, which is why they're uniformly pale. He did replace the outer six primaries during the pre-basic molt. Counting from left to right, you can see that the outer six flight feathers are darker than the next seven. Three of those seven feathers are primaries and the next four, squarer-tipped ones, are secondaries. After that come two secondaries that were replaced during the pre-alternate molt along with the three replaced tertials. It also seems as though he retained the outer 7 greater coverts and the 2 outer median coverts. All in all, a much more interesting wing than I thought when I photographed it earlier today.