Different species of birds molt, or change feathers, in different ways. The Prothonothary Warbler (PROW) is in the group of passerines, or “perching birds”, most of whose members molt once/year. Sometimes a molt is partial, that is, not all the feathers are changed. In some species, like the PROW, and American Redstart, Orchard Oriole, Indigo Bunting, Rose-breasted Grosbeak and a few others, the male’s plumage changes as it ages, and we can tell how old an individual is, of these species, up to their third year. In other species and many females, we can tell how old they are up to their second year by the shape of, and the amount of feather wear on, the tail feather tips. These birds breed during “our” spring, but we age them on a calendar year basis, one that starts on January 1. So, a bird we catch or see in its first “year” we call a “hatch year” (HY) bird, or immature. After January 1, it “becomes” a “second year” bird, or SY. Males of some of the above mentioned species will continue to look like a young bird, or like the females, whose plumages are much duller, through this second year. They are nonetheless reproductively mature and capable of breeding, if they can attract a mate. If these birds survive this second year and return the next, they are dubbed “after second year”, or ASY.

In the beginning: when a bird of this group (perching birds) hatches, it is naked. Soon, natal down feathers begin to grow in – these are generally gray, fuzzy feathers. But they often only last a week or so, and then are replaced by feathers that look more like the feathers we are used to. These are called juvenile feathers and the stage is called the juvenal plumage. The young birds will fledge from the nest in their juvenal clothing. The image below shows one such PROW, soon after leaving its nest cavity.

Changing fashions: like kids everywhere, even young birds must have the latest. Within a few weeks of leaving the nest with their juvenile feathers, the young PROW’s begin another molt. This time, it will be a partial molt – it will include the body feathers, and some of the smaller wing feathers, the one’s that cover the upper shafts of the main flight feathers. The images below show such a tyke. These birds will acquire feathers that are very much like the adult female. This is considered the “basic” body plumage, so this event is termed the pre-basic molt (or post-juvenal molt), and not surprisingly, the result is the basic plumage. Since some PROW will nest twice, thus into July, we see some young birds molting then. By early August though, it appears that all the kids have new clothes. Technically, these birds are no longer juveniles, and we call them immatures. They will now wear these feathers until next June/July, at which time a complete molt of body and flight feathers will occur.

The plumage of most immature birds, especially in this group, is very much like the females, and thus it is often impossible to tell males from females. In PROW, the amount of white on the underside of the tail feathers usually differs enough in young birds for us to be able to tell – but not always. Males show more white on feathers overall, and also show white on more feathers (usually 10/12). Females usually show white only on 6 (the outer 3 on each side) of the 12 tail feathers. As the bird’s age, and molt, the differences become greater, and along with some of the other color patterns, we can more easily tell male from female. The images below show some tail patterns. The first is a young female. The other image shows 3 adults lined up: female, SY male, and ASY male.

In this group of birds, the tips of the tail feathers of HY/SY (younger) birds are more pointed than “full adults”. Remember that the HY will become an SY on January 1. So, when these young migrants return, they still have the pointy tail feathers, albeit now a bit more worn. For birds which look like older birds in plumage, we can use this character to age the bird in the hand, during banding. For example, in PROW, a young SY female will look like an ASY female in overall plumage color/pattern, but those tail tips will differ. The images below show the pointy HY/SY tail feather and the more truncate tips of birds that have molted the flight feathers at least once.

So, if you’re still reading, and following along (and I applaud you for either or both), you may be wondering what happens to all this naming of stages in August. Here, any SY birds have now molted into the more “definitive” (final adult) plumage, but we’re still in the same calendar year. So the conundrum is we can no longer assess an “SY” bird, and yet we know many of them ARE. It all can get very confusing, which is why Becky and I stop banding on July 31. Who needs this? What with all the other issues in our lives …………..……… just kidding! Any such late summer/fall adults are scored as either HY or ASY, but when we capture adults whose age we cannot assess reliably, we simply call them AHY, for “after hatch year”.

But during the breeding season, we can age and sex most of these birds. The images below show 3 adults: a female, a SY male, and an ASY male; and another of just two males, SY and ASY. Notice the completely olive wash on the female’s head, the partial olive wash on the SY male, and the clean break of the dorsal color vs. the yellow head of the ASY, older male. The SY males, as you can see, show a fair amount of olive coloring in the nape and crown region. The amount varies among individuals but is consistently seen in some amount in this age class. Again for females, we have to rely on tail shape. For data recording purposes, whether while banding or studying specimens, if we cannot determine an exact adult age, then again we refer to the bird as an AHY. We know it’s an adult, but not “how much” of one.

All of the images of males on the main PROW page are ASY birds. The perched “female” is a young bird (HY), whose gender cannot be determined from this angle.

All photographs were taken by John Gerwin.