first calendar year: August

Last update of this page: 27-09-05 15:09

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 the Maasvlakte and 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. 

Such groups may consist of up to 3.000 LBBG by mid September 2000. 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 probably. Nevertheless, 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 a reliable feature 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, 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 2^nd 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.

Nevertheless, direct comparison with michahellis shows on average fresh (warm/dark tinged) second generation feathers on LBBG 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 heavily worn. 

Summarizing, most LBBG juveniles can readily be identified by:

• the dark inner primaries,
• 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 black in the tail and 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, to name jizz and contrast in dark base and pale fringes may be indicative. Where most fuscus LBBG lay eggs almost two months later than average Dutch LBBG, it is highly unlikely to find such advanced covert and scapular moult in fuscus juveniles.

Maasvlakte: 1cy LBBG research
research by GRO

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 Gull.

Taking these factors in account, we measured category II & III chicks less intensive. 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 two plots, from earlier research by Arie Spaans at Terschelling. In his paper, he provides several plots, of which two are presented at this page, below. 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.