American Herring Gull (smithsonianus)

(last update: July 12, 2015)

American Herring Gull 2cy June - Complete moult in summer months

Herring Gull is a widespread breeding bird with very restricted post-juvenmile moult, which makes it possible and easy to construct a picture of the seuqence of the complete moult in summer. Lesser Black-backed Gull fuscus can show large individual variation in the extent of post-juvenile moult, making it more difficult to get this picture of the complete moult. First a summary of the plumage development to set the starting point of the complete moult: back to 1CY birds.

Above: Herring Gull X.V 1cy (argenteus), August 22 2012, Katwijk, the Netherlands. Completely juvenile plumage. Ringed in the Netherlands. Picture: Mars Muusse. Below: Herring Gull A5UW 1cy (argenteus), November 09 2008, Boulogne-sur-Mer, France. Mantle and scapulars replaced for 2nd generation, except single rear scapulars in lowest row. All coverts, tertials, and flight-feathers still juvenile. Ringed in England. Picture: Jean-Michel Sauvage.

Complete moult in summer months - back to 1CY

To know what exactly goes on in the complete moult, its best to first have a comprehensive picture of what the starting point exactly is. We have checked Herring Gulls in the field to score 1st winter birds (15 November - 15 March) on extent of scapular moult, and replacement of tertials and wing-coverts. The main findings are:
- moult in scapulars show large individual variation, which has much to do with origin of the native colony. Argenteus from southern locations have more extensive scapular moult than nominate argentatus birds which hatched far north in Scandinavia. You can visit the webpages of 1Cy November - 2CY April to get an idea of this scapular moult and many illustrative images of ringed birds from all directions.
- moult in scapulars continues at very low pace throughout winter months. It is commonly seen in November birds to show the last juvenile scapulars in the rear lowest row. By March, these feathers are replaced and by that time it is common to see some fresh feathers in the upper scapular rows, which now are 3rd generation. Note that P1 is still present in these birds, so by definition the complete moult has not started yet.
- Herring Gull consistently show no moult in wing-coverts nor tertials in 1st winter, of course with the rare exception of single birds that do. Checking larger flocks of 1st winter birds will reveil that about 2%-3% replace single feathers (hence the term 'moult' should perhaps not be used, but this discussion better takes place at internet fora). Statistically, this implies you will find 1 out of 35-50 birds to have replaced covert(s) or tertial(s) for 2nd generation. We found this in both argenteus and argentatus.

Above: Herring Gull A7JG 2cy (argenteus), March 05 2011, Boulogne-sur-Mer, France. Mantle and scapulars replaced for 2nd generation, except single rear scapulars in lowest row. Last replaced scapulars still have buffish tone. All coverts, tertials, and flight-feathers still juvenile. Ringed in England. Picture: Jean-Michel Sauvage. Below: Herring Gull Y.ALM 2cy (argenteus), May 08 2010, Leiden, the Netherlands. Complete moult just started: P1 missing (A). All coverts still present. Ringed in the Netherlands. Picture: Maarten van Kleinwee.

Complete moult in summer months - first phase replacing primaries

In early spring, 2CY birds drop P1, which is the initiation of the complete moult in the summer months. This complete moult will include all feathers, including large flight feathers. Most of 2CY birds, especially argenteus and argentatus, will follow a strict 'ordinary sequence' in this, starting with P1. By the time P6 is halfway in July, they start dropping the outermost secondary (S1) and central tail-feathers (R1). Birds look very untidy in this period, missing central primaries, and another gap in the outer secondaries and aslo a moult gap in central tail-feathers. By late September all these gaps are filled again in argenteus. However, N Scandinavian argentatus may be later. It is interesting to see that field surveys from Finland show that 2CY Baltic argentatus are not behind when compared to argenteus; on the contrary, they may be slightly advanced and ahead in moult. This is also true for another northern spcies in Europe: Larus marinus.

After P1 is dropped, the moult wave slowly progresses through the primaries outwards. P2, P3, and P4 are dropped, and at the same time the tip of P1 becomes visible (in fact, it is the growing new 2nd generation feather pushing out the old feather from the pin). When P1-P2 are growing another feather tract is involved in the process: the covert panel. First coverts to be dropped are outer median coverts, followed by central median coverts and outer lower lesser coverts. Two pictures below illustrate this moult stage.

Above: Herring Gull 2cy (argenteus), June 08 2013, Katwijk, the Netherlands. P1-P2 new, P3 growing. Outer and central median coverts missing (A) and outer lower lesser coverts missing (B). Picture: Mars Muusse. Below: Herring Gull 2cy (argenteus), June 08 2013, Katwijk, the Netherlands. P1-P2 new, P3 growing. Outer and central median coverts missing (A) and outer lower lesser coverts missing (B). Picture: Mars Muusse.

Just like in the primaries, the new 2nd generation coverts 'push out' the worn old juvenile feathers. Within a few days, the tips of the outer median coverts become visible, and also the outermost new lower lesser coverts. This is illustrated in the picture below. Meanwhile, primary moult progresses of course, and also more coverts are involved: when the new median coverts are visible, the outer and central greater coverts are dropped. These feathers fall out almost simultaneously leaving a large moult gap. Another moult wave starts from the upper tertials. As the greater coverts are dropped, the bases of the secondaries lying underneath become visible, including the broad white base of the secondary shafts. In this phase, all secondaries and tail-feathers are still present.

Above: Herring Gull GR 15426 2cy (argenteus), June 01 2012, Boulogne-sur-Mer, France. Inner primaries replaced. Outer and central median coverts replaced, feathers show barred pattern on brown-grey tone. Ringed in England. Picture: Jean-Michel Sauvage. Below: Herring Gull 2cy (argenteus), June 10 2013, Katwijk, the Netherlands. P1-P4 new, P5 growing. Outer and central median coverts replaced, feathers show barred pattern on brown-grey tone (A) and all outer and central greater coverts missing (B) exposing secondary bases underneath. Outermost greater covert tips just visible below tips of new median coverts. Picture: Mars Muusse.

Moult in the coverts is initiated in the outer wing, but another moult wave start from the inside. These two moult waves meet at about about the 6th-8th covert in the row. This is most obvious in the last phase of the greater covert moult when the two moult waves meet.

Complete moult in summer months - second phase replacing secondaries and rectrices

When P5 is fully grown and P6 is growing, another obviously visible phase in the complete moult becomes visible: replacement of secondaries and rectrices (tail-feathers).
It's mid-July now, inner primaries P1-P5 are neat fresh 2nd generation feathers. The complete row of median coverts is new as well. The row of lower lesser coverts may sometimes show just single feathers growing, but otherwise has been renewed completely as well. But the rest of the bird is really messy and untidy: The greater covert row has new feathers in the outer wing and the moult wave progressing inwards. Another moult wave started from the inside, progressing outwards and the two will meet at about GC6-GC8. GC6-GC8 are the greater coverts which can be seen as small group of very worn juvenile feathers in July, the last to be dropped in the greater covert row.
The upper tertials are new, but central tertials are missing and growing, the moult wave progresses downwards. The very small feathers in the lesser covert region are a mix of old juveniel and fresh 2nd generation feathers, and since these feathers are small, real moult gaps are hard to detect.
By now the central tail-feathers are shed (see picture below). Moult in rectrices progresses 'centrifugal', which means the central feathers are dropped first and moult progresses outwards, often skipping R5, which normally is the last tail-feather to be dropped. Birds look very fluffy now, with missing tail-feathers and just a bunch of upper and undertail coverts left.
Now the moment is there to start dropping secondaries as well. First the outermost S1 is dropped, and starts to grow. S2 follows and subsequently more and more secondaries will follow, at shorter intervals leaving a rather large moult gap in the central secondaries. The six tertials received their own name 'tertials' but can also be classified as 'innermost secondaries', summing up to 24 secondaries; as they consist of feathers from the same tract. Therefore it is no surprise these two moult waves also meet halfway, at about S16-S18.

Above: Herring Gull CP.AX 2cy (argenteus), July 15 2009, Boulogne-sur-Mer, France. P1-P5 replaced, P8-P10 still juvenile. Outer greater coverts growing, central greater coverts missing. Another moult wave starts at innermost greater coverts, these two moult waves will meet at greater coverts #6-#8. Upper tertials growing as well. Ringed in Belgium. Picture: Jean-Michel Sauvage. Below: Herring Gull 2cy (argenteus), July 18 2013, Katwijk, the Netherlands. P1-P5 replaced, P8-P10 still juvenile. Outermost secondary S1 missing (A). Outer greater coverts replaced, central greater coverts growing. Inner moult wave progressing outwards, these two moult waves will meet at greater coverts #6-#8 (B). Central tail-feathers have been shed (C), just the upper and undertail coverts are left. Picture: Mars Muusse.

Initiation of rectrices moult and secondary moult may be inverse in some birds, hence starting with secondaries and dropping tail-feathers soon after.

When the flight-feathers are moulted, nearly all wing-coverts are already new. Maybe only a few feathers in the carpal edge have to be replaced. Flight-feathers are large feathers and probably require much energy to grow. Also, missing feathers influence manoeuvrability. Next two pictures below show how the juvenile secondaries and rectrices are replaced. This process develops quite rapidly; By the time P10 is still growing, all tail-feathers and almost all secondaries will be new.

Above: Herring Gull 2cy (argenteus), August 20 2012, Katwijk, the Netherlands. P1-P7 replaced, no juvenile primaries left. Greater covert row closed (A) at #6-#8. Central tail-feathers growing, one feather still old juvenile (B). Upper tertials replaced, lower tertials missing. Secondaries S1-S3 new (C), central secondaries missing. P1 is (D). Picture: Mars Muusse. Below: Herring Gull 2cy (argenteus), September 12 2012, IJmuiden, the Netherlands. Last stage of complete moult: P1-P9 replaced, P10 growing (B). S1-S12 replaced and tertials replaced; last gap in secondaries almost closed (A). Tail-feathers all new 2nd generation. Picture: Mars Muusse.

When P10 is fully grown, this is the end of the complete moult. Interestingly, many Herring Gull have a partial moult again in autumn, of which the start may overlap with the last stage of the complete moult. In this partial autumn moult, coverts and tertials which were replaced already in early summer, are replaced for another time. These coverts and upper tertials are replaced two times in one year.

Above: Herring Gull A5UW 2cy (argenteus), September 04 2009, Boulogne-sur-Mer, France. Again A5UW, now showing the initiation of the partial autumn moult. Growing upper tertials (A), median coverts (B)and innermost greater coverts will be replaced again, and in turn appear fresh. Last aquired feathers lacking obvious barring, more adult-like grey. Partial autumn moult started while P10 not fully grown. Ringed in England. Picture: Jean-Michel Sauvage. Below: Herring Gull A3CV 2cy (argenteus), November 26 2006, Boulogne-sur-Mer, France. When the partial moult is finished as well, many Herring Gull show a contrast between barred 2nd generation coverts and more adult-like 3rd generation coverts (normally upper tertials, inner greater coverts and many median and lower lesser coverts). These are last replaced, hence fresher with grey and buffish tones by November, but will bleach later in winter. Ringed in England. Picture: Jean-Michel Sauvage.

banded 2CY HEGU in June

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