Australia - Feather Molt - 80 Mile Beach WA, November 15, 2007
If dealing with the effect of heat on bird and team is the first major lesson of trapping in northwest Australia, the complicated wing molt of tropical non-breeding waders is the second. The heat, sun and wind wear down everything. For example the engraved leg flags on red knots and other shorebirds in the Delaware Bay have lasted for 5 years without significant fading of the unique alpha-numeric characters that indicate an individual bird. Here in Roebuck Bay, flags can fade within a year. What the elements do to flags, they also do to feathers. The impact is a highly evolved system of molt that allows birds to replace worn feathers and migrate when they need to with maximum aerodynamic efficiency.
Alice showing a newly-banded Bar-tailed Godwit
An adult Red Knot in the middle of primary molt
An average shorebird (everything varies by species) gets its first set of flight and body feathers the month after it is hatched. In July and August, young birds fly south to wintering areas which, for Red Knots in the Western Hemisphere, could be all the way to Tierra del Fuego at the southern tip of South America. In this, the Australia-Asia Flyway, a young bird flies to Roebuck Bay or 80 Mile Beach from its arctic breeding habitat, arriving in October or November. Most may skip the next migration north and may stay near the wintering area or make a partial northward migration. Although some start putting on a new set of feathers shortly after arriving in Australia, most birds wait and molt their juvenile flight feathers through the next July-August after their feathers become worn. Interestingly these same birds (now two-year olds) may start a second molt in the fall, so that when they are in-the-hand you may see feather molt starting at the inner primaries (at mid-wing), while looking at relatively new, fully grown outer primaries (at wing tip). The adults coming to Roebuck in the fall begin their molt in the fall. In any case, all molt is completed by January at which time all the second-year birds and adults use their new flight feathers to take them back to the Siberian or Alaskan Arctic.
Clive giving a lecture on molt in non-breeding shorebirds. Note graph of molt score on the easel
An adult Red Knot at least 3 years old in active molt (below), and a second year bird (above). The second year bird would have stayed in Australia throughout the northern summer of 2007 and started its primary molt long before the adults returned. Therefore its molt is in advance of the adult's; 9 of its 10 primaries are fully-grown and the outer 10th primary is about three quarters grown.
Another second year Red Knot in the midst of a second (or "replacement") molt -- unlike the adult the outer-most primaries (P9 and P10) are dark and relatively fresh while P8 is not yet completely grown. Note the primaries and secondaries look uniformly dark and fresh.
A science has blossomed around molt, not just for primary flight feathers (at the outer end of the wing called “primaries”), but including secondary flight feathers (at the inside of the wing called “secondaries”), flight feather coverts, body feathers, and tail feathers. They all contribute to a story that can reveal the bird’s recent history. There are suspended molts (a stressed bird may stop molting and is left with some new, completely grown feathers and the remainder are old feathers). There is a complicated naming system for primary molt ( P1 to P10 – inner primaries to outer primaries). There is a nomenclature describing the number of feathers in one of five growth categories that is bizarre but elegantly simple. It is written as one of the five growth categories raised to the power equal to the number of feathers in that category: 55, 42, 11, O2 = a bird with 5 feathers fully grown (category 5), 2 feathers that are more than 2/3 grown (category 4), 1 feather that is in "pin" (no feather has erupted), 2 feathers that are old (category "O" for old). Add the superscripts together and it equals the total number of primaries (10); multiply the superscripts by the growth stage, add all the products and you get a molt score. The molt scores can be plotted to describe the overall molt stage of birds at any time during the non-breeding period.
I admit I find all this hard to explain, but believe me it is truly hard to understand. Nearly everything I’ve just written varies by species, year, conditions, etc. But some people know molt sufficiently to be comfortable with any new combination of feather wear, color and growth stage to tell a story -- Clive, Chris, Roz and David Melville to name a few. In fact, it’s a bit like chess or a good murder mystery, a kind of game for intelligent people to describe a bird's past. Most of the veterans of these expeditions, like Humphrey Sitters, know molt well.
Molt is not usually an issue in the Delaware Bay because most of the birds are O10 (feathers are all fully grown and old after having molted the previous fall). However, molt has become key to unraveling the southbound flight of Red Knots on the east coast of the US. This August we trapped Red Knots in Stone Harbor, New Jersey, and they were in the midst of primary molt (55, 41 11 O3 for example). Additionally, most of the recaptured birds were from the catches we made in Florida in the last two years. One week later, we were trapping in Mingan, Quebec, a major southbound stopover for red knot, located on the north shore of the Gulf of St. Lawrence about 800 miles north of Stone Harbor. This stopover was recently discovered by Yves Aubry of the Canadian Wildlife Service. With Yves, we trapped a small group of knots that were all 010 -- they had not yet started their molt. In other words, they were on a completely different molt schedule than the Stone Harbor Birds. Also, those bird carrying leg flags in Mingan were from South America or Delaware Bay and none from Florida. Thus, molt and resightings suggest that the birds in Mingan and Stone Harbor represent different non-breeding populations – those in Mingan go to South America, those in Stone Harbor go to Florida. In this way, molt can often unravel complicated bird life histories.
Google Earth map of US East coast showing Stone Harbor, New Jersey, US and Mingan, Quebec, Canada