first calendar year: August

In August most Dutch LBBG leave the colony at the Maasvlakte, Rotterdam (for locations, see the distribution page). By the end of August, 99% of the adults and their juveniles (approximately 70.000 birds) have left the colonies of Maasvlakte & Dintelhaven and they start migrating south (at least leave the immediate feeding grounds around the colony). For a while, they gather at the beach near the colonies, in mixed groups of different age-classes (image below). 

Identification from michahellis

Such groups may consist of up to 3.000 LBBG by mid September. In these groups the races graellsii and  intermedius are both present. Naming races in juvenile plumage is very difficult, and often only possible when birds are banded / ringed or fed by ringed parents. From mid-October the number of juveniles steadily decreases to several 100s and most Dutch LBBG juveniles have to be sought after in Portugal or further south.

Although racial separation of juveniles is very difficult, juvenile LBBG is readably different from other white-headed gull juveniles, Yellow-legged Gull (L. michahellis) coming closest. When inner primaries and tail together with moult score and abrasion of scapulars and wing coverts are seen well, juveniles are quite distinctive.

For long it's known that moult score and quality (condition) of body feathers is one of the more reliable features to distinguish michahellis from LBBG; LBBG being the later hatched, northern breeding species of the two. Subsequently, when juvenile LBBG and YLG are seen side-by-side in August and September in the Netherlands, YLG often stands out with more abraded scapulars, tertials and wing coverts, the fringes already worn, centres slightly bleached and often most scapulars and some coverts moulted to second generation feathers. 

But as the table below shows, some LBBG do moult quickly or early, at least well before departing to the wintering grounds. Half of the sample of juvenile LBBG replace (at least some) mantle feathers and scapulars, as early as "still in the colony and begging for food". The table below shows the results and some examples are shown in the images right.

When directly compared with michahellis, moulting graellsii show on average fresh (warm/dark tinged) second generation feathers; where michahellis already shows more bleached (i.e. paler) upper-parts, strongly contrasting with the coverts and tertials. Juvenile coverts, scapulars and tertials in michahellis juveniles often show fringes and tips which are already worn. 

Summarizing, most LBBG juveniles can readily be identified by:

• the dark inner primaries, broad tail-band, dark shins, general shape,
• dark centred tertials with narrow straight white fringe,
• pretty neat scapulars and tertials well into September, early October, showing no extreme wear,
• dark brown under-wing coverts,
• dark greater covert bar created by the base of the greater coverts, especially the outer greaters,
• dark chocolate brown centres of scapulars and coverts,
• more barring on upper-tail coverts.

Separating sub-species of graellsii, intermedius, the intermediate Dutch intergrade and fuscus is hard to do in juveniles, although some general features (jizz and contrast in dark base and pale fringes) may be indicative. Where fuscus LBBG lay eggs later than average Dutch LBBG, it is highly unlikely to find advanced covert and scapular moult in fuscus juveniles.

Maasvlakte: 1cy LBBG research
research by GRO

Paragraph continuation from May, June & July sections).

Early in the breeding season it was relatively easy to monitor chicks on a regular basis, by measuring them two or three times a week. This allowed accurate estimations of growth rate for the pulli of Category I (hatched before June 10). 

For category II & III, which hatched late June, such regular visits to the nests would have much impact on the colony for two reasons. A: early hatched chicks would run away large distances through the colony, making them vulnerable for attacking adults. And B: recently hatched pulli and eggs would be left unattended and nests could be destroyed by predating adults and Herring Gulls.

Taking these factors in account, we measured category II & III chicks less intensively. Immediately after hatching, birds were measured and tie wrapped. Only three weeks later, birds were measured again when the darvic rings were attached. Some category II & III birds were measured occasionally in between these dates.

For some individuals, two measurements were not sufficient to allocate birds to a certain category, especially not when hatching was not confirmed. For these individuals we used plots, from earlier research by Arie Spaans at Terschelling. In his paper, he provides several plots, of which two are presented here. Spaans estimated growth graphs for LBBG juveniles in 1992 and 1993. 1992 proved to be a year with food shortage, in 1993 food supply was not a limiting factor in daily growth rate. Growth rate was measured by body mass increment and wing length. We used these graphs to calculate hatching date for Maasvlakte birds in 2004, only when field measurements didn't allow precise hatching date estimations.

Results

This section will follow soon.

- Some juveniles show fresh feathers in the wing-coverts and scapulars. How can this be explained: late 1st generation or already 2nd generation?

- The presence of such 2nd generation (like) feathers, is it depending on hatching date?

- If post-juvenile moult may start in the colony, what is the range of this moult?