Moulting: A Key ID Feature Flashcards
The moult patterns visible in a gull's plumage often provide invaluable clues about its identity. In this deck, we'll learn about when throughout the year a gull moults, how this cycle shows up in its appearance, and the clues to look for to guide you towards a more confident ID.
What’s the significance of moult patterns when trying to identify gulls?
Different gull species have diagnostic moult patterns that can help make identification easy!
Paying attention to moult patterns is the ultimate birder’s hack for identifying gulls, since other markers like plumage can be confusing and misleading.
In most cases, how does the large gulls’ head plumage differ between summer and winter?
In winter, the large gulls—like glaucous-winged and herring gulls—develop a smudged, dirty-white hood.
In summer, adults develop a clean and crisp (unmarked) white head (to look good for the breeding season).
Aside from a changing head plumage, what other traits do breeding gulls typically exhibit?
Their “naked parts”—bills, legs, and skin around the eye—become bolder and more vibrantly colored.
For example: glaucous-winged gulls’ bills turn a brighter yellow with a bold red dot and their legs go from pale to deep, dusky pink.
On average, how long does it take the large gull species to achieve full adult plumage?
Adult plumage being: crisp white and gray (and black) feathers without any gray or brown mottling.
Four years
First-year gulls exhibit exclusively (mottled) brown plumage.
Second and third-year gulls exhibit brown, gray, and white plumage with an increasingly gray-and-white appearance as they age.
Four-year gulls exhibit pure white and gray plumage.
What stressors cause wear-and-tear on a bird’s feathers?
- Sun bleaching
- Wind
- Wind-borne sand
- Use
Describe the first set of feathers grown by young gulls.
Pay attention to quality and color
Young gulls develop ALL of their feathers at the same time—and they do so in a hurry because they only have a matter of weeks before they are required to fledge and become independent. These first feathers are mottled/brown in color and of a lower quality than the more mature feathers they will develop over the ensuing seasons. This makes them more prone to wear-and-tear.
The next time you see a flock of gulls, seek out the young ones and see if you can notice any obvious signs of wear-and-tear on their feathers.
How often do gulls moult?
Gulls replace all of their feathers once a year towards the mid- to end of summer.
They undergo a secondary partial moult in the middle of winter of the feathers that are the most exposed to the sun’s damaging UV rays: the mantle.
The exception to this rule is Franklin’s gull, which undergoes two complete moults each year. (Because they migrate between two food-rich environments.) Consequently, as a bird with three programmed “age groups”, they achieve full adult plumage at 1.5 years old.
Large gulls typically take four years to reach full maturity. But how long does it take small and intermediate-sized gulls to achieve maturity?
An example of a small and intermediate gull would be the Boneparte’s and ring-billed gull respectively.
Small gull: 2 years
Intermediate gull: 3 years
Small gull are said to have two age groups, each with their own unique plumage: juvenile and adult plumage.
Intermediate gulls have three age groups: juvenile, intermediate, and adult plumage.
Large gulls have four age groups: juvenile, two intermediate years, and adult plumage.
How can the timing of a bird’s moult assist with ID?
If it is early winter (November-December) and you see a gull in full first-year plumage, you can be pretty sure that it is a high Arctic breeder, like Thayer’s, Herring, or Glaucous gulls.
If, however, you notice that the gull has already begun moulting, it likely originates from further south (like California or Mexico), making it more likely to be a Western gull or similar.
This clue loses its usefulness by late winter, when both high Arctic and low latitude breeders will both have fully moulted.
What are some examples of gulls that breed in the high Arctic?
- Thayer’s gull
- Glaucous-winged gull
- Herring gull
What is the mechanism behind the different moult timing of high-Arctic and low latitude breeding gulls?
High Arctic breeders have their chicks later in the summer (July), when it is warmer. This means that first-year gulls will only start moulting towards the end of winter, from December onwards.
Low latitude gulls, on the other hand, can probably start having chicks as early as February (because it’s much warmer) and so these juveniles can start moulting as early as August.
Because gulls mature at approximately the same rate, a Thayer’s gull (Arctic breeder) will only start moulting in December/January, while Western gull (California/Mexico) will start moulting from August onwards, even though they are born in the same calendar year.
In other words, the two species moult several months apart. Knowing this—and knowing which gull species breed where—provides a useful ID clue!
