It seems I get the most inquiries about molt and ageing of Indigo Buntings, followed closely by Rose-breasted Grosbeaks (RBGRs). These two Carduelines share a few things in common: 1) they are both popular and well-studied birds in eastern North America that are often captured at MAPS stations; 2) they both occur rarely but regularly as migrants or vagrants in western North America as well, where they generate excitement; 3) plumages of adult (ASY) males of both are stunning (and also females, but more nuanced), and those of young or second-calendar-year (SY) males are especially varied; 4) despite being common and well-studied, nobody is sure where either of these two species undergo their molts, but it does not seem to occur on breeding grounds (see here); and 5) they hybridize with Lazuli Buntings and Black-headed Grosbeaks, respectively, in the middle of the continent, which adds further fun to the various plumage equations. I'll add one more thing about Rose-breasted Grosbeaks (rarely in Indigo Buntings): they are capable of undergoing incomplete second prebasic molts, allowing the ageing of some birds up here in the spring and summer as in their third calendar year, or TY (see here, p. 58, for an example of an SY RBGR in fall, which will remain like this and become a TY on January 1st). We'll leave Indigo Bunting singing on the roadside for now, but see here if you want to learn more than you really care to about their molts and plumage variation. To analyze our RBGR plumages, I will again and forever turn to the Cornell Lab of Ornithology's Macaulay Library, where at the time of this writing there were >110,202 images of RBGR catalogued, of which >50,051 were taken in May, in turn of which >1,038 were taken in Colorado in May (see here for more on using the Macaulay catalogue to study molts and plumages).

The bird that visited Meredith's yard in late May 2021 (photo 1) is a fairly "standard" (if this is even possible) SY male RBGR, although we do have to be careful to make sure it is not a TY. First, note that the outer greater secondary coverts are black to blackish. These are formative, contrasting as a molt limit with the shorter and browner juvenile alula and primary coverts. The primaries and secondaries are also juvenile, save for the replaced tertial that can barely be seen along the top of the secondary tract. I did wonder a bit about why the secondaries appeared much browner than the primaries, and if this could be a TY that had retained secondaries (as in the fall SY on p. 58 of the article linked above and see also below), but I think they are browner due to a combination of their being more exposed to the sun and to the primaries being naturally a bit darker (due to more melanin in the feathers), resulting in what we call a pseudolimit, as we can be fooled into thinking these are true rather than false molt limits on older (ASY) birds. What about the rectrices? For these I searched the Macaulay Library for eBird Hotspot "Big Salt Wash" (Mesa County, CO) in May 2021, as Meredith suggested, and in this image (photo 2) of the same bird you can see that the outer rectrices are blackish with large white patches. Juvenile rectrices are brown with smaller white patches, as seen on this September hatching-year (HY) male from Colorado (photo 3- the pink underwing coverts make this a male, as females of all ages have yellow coverts). So these were replaced in the May bird and are formative. None of the images of this bird allowed us to see how many tertials and inner secondaries were formative but I suspect at least the three tertials given that the rectrices were replaced. Molt extents correspond to each other among tracts, as we show for those of the Willow Flycatcher here.

But wait, if you consult the 1997 version of Identification Guide to North American Birds, Part 1, those new black wing coverts, tertials, and rectrices are considered first alternate feathers, but here I say they are formative. What gives? Well, this is the result of a long and contentious story having to do with molt terminology, and the fact that, in 1997, I had not quite switched over in my brain from the life-cycle approach of the Europeans to the Humprhey-Parkes (H-P) approach, something our simple but transformative revision of H-P six years later (Howell et al. 2003), defining preformative molts, assisted me to do in a big way.

For RBGRs, the preformative molt usually begins somewhere in the breeding range, is suspended over migration, and then resumed on the winter grounds in the late fall and early winter. An interesting aspect to this is that formative feathers replaced in male RBGRs before migration are brown, and then these get increasingly blacker for those replaced on the winter grounds as the hormones controlling feather pigment in males gradually mature. Thus, on our May Colorado bird, those brown lower back feathers are actually formative, replaced the previous August or so in the summer range, and then the blacker wing coverts, tertials. and rectrices were replaced in November or so on the winter grounds. They are both considered formative because they all replaced juvenile feathers and as based on the molt sequence: generally, head-to-toe for body feathers, followed by wing coverts from inner to outer, then teritals and rectrices, in each case beginning with the middle feathers. Thus, a molt is defined by a given replacement sequence, which can actually overlap the next molt, recognized by the start of a new sequence. I expound upon this sequence-based approach to define molts here, using male Summer Tanager as a similar example of a species that often suspends the preformative molt for fall migration.

So do RBGRs even have prealternate molts? I wondered for a time, as the black head and rose breast of both SY and ASY males could possibly be a result of the wearing of fresher brown feather fringes, as occurs in Snow Bunting and some other species. As an example, could the brown definitive basic head feathers of this adult (AHY) male RBGR from California in September (photo 3) result from fringing that wears off to produce the brilliant ASY males we see in spring, e.g., this May bird from Colorado (photo 5)? For SYs, could the head and breast feathers of our bird have been replaced during the preformative molt on the winter grounds, after hormone maturation, and be formative? In both cases the answer is no. For the adults, I had to check the feathers of specimens collected in fall to make sure the basic head feathers were not black with brown fringes, and instead many are mostly or entirely brown (as seen in the September AHY male in photo 4), so impossible to turn into the shiny black definitive alternate feathers of spring. For our SY, recall that sequence of body feathers generally goes 'head-to-toe.' Thus the head feathers would have been replaced before migration, and the formative head feathers would be brown, similar to those of the lower back and in our September HY male from Colorado (photo 3). In the May 2021 Colorado bird (photo 1 above), therefore, the black head and upper back feathers, as well as the red and white breast feathers and, I think, those two longer and blacker greater coverts above and to the left of the lower white bar across the primaries, are first alternate, as defined by molt sequences, and I think this extent is likely typical of replacement during both the first and the definitive prealternate molts. For example, greater coverts are usually replaced from inside out so it makes sense that those two inner feathers replaced formative rather than juvenile coverts, and note also that they are blacker than the outer coverts. This is all cleared up in the Second Edition to the Identification Guide published last year. And, by the way, all of this applies to Black-headed Grosbeaks as well!

The plumages of SY male RBGRs in May vary tremendously in appearance, as a result of varying extents of both the preformative and prealternate molts, as can be seen from our 1,038+ Colorado birds, most of which are SY males, a bit off course at the edge of their normal migratory and breeding ranges. Examples of less-mature looking SYs are here and here, whereas more-mature looking SYs are here and here. In all cases, though, note that the outer primaries, outer secondaries, and primary coverts are brown and worn, juvenile feathers. Which brings us to our second bird observed by Meredith this year (see main photo at top of page.) Although the photos are not as clear as for our 2021 Colorado May SY, we can see a similar bird to the more mature-looking SYs linked above, with mixed brown and black wing feathers. So why is the 2023 bird actually a third-year (TY) and not an SY? Note that the primary coverts and primaries look black in this image. As these feathers are not replaced during the preformative molt this color indicates replaced basic feathers. Another thing to note is the size of the visible white patch at the base of the primaries, which is almost diagnostically larger in ASYs (14-24 mm from the primary coverts) than in SYs (4-14 mm). So this is a bird that had replaced primaries and primary coverts but had retained brown juvenile secondaries following the second prebasic molt at a year of age, similar to the fall SY (to become a TY) discussed on p. 58 here, in which the juvenile secondaries s3-s6 were retained, the last feathers replaced during a complete prebasic molt. So this 2023 individual is not the same individual that visited Meredith's yard in 2021, because that bird would now be 4 years old and would have no juvenile feathers.

Which exact juvenile feathers were retained by our Colorado May TY? To determine this I went to the video and freeze-framed some images for examination. With a bit of imagination (and perhaps hallucination) I infer that 1-2 outer primaries (p8-p9) and corresponding primary coverts, all secondares except the tertials (s1-s6), and three rectrices on each side of the tail (r4-r6) are retained juvenile feathers, contrasting with much blacker second-basic feathers. These are, again, the last feathers replaced during a normal complete molt and the most likely, therefore, to be retained. I think this is a record number of juvenile feathers I've seen retained on a TY passerine in North America! Why I love this "Ask Peter?" series and other questions from MAPS banders and others, as I never know what I will end up leaning from it.

PS: Just after writing this Peter found a vagrant, singing male Rose-breasted Grosbeak atSutro Heights Park in San Francisco (here). The lighting was bad and Peter concentrated more on photographing it than ageing it, mistakenly assuming one age when it turned out to be another. Better photos were taken by another observer here. Can you age this Rose-breasted Grosbeak?