adult: August

Last update of this page: 25-07-05 18:10

adults in the colony

In the Maasvlakte colony many adults try to raise two juveniles. As soon as the fledglings can move, the family tries to find a more comfortable place outside the colony, often at the beach or the near fresh water lakes. By the end of August, 99% of the adult LBBG from the Maasvlakte and their juveniles (approximately 70.000 birds) have left the Maasvlakte and start migrating south (at least leave the immediate feeding grounds around the colony). For a while, they gather at the beach, in mixed groups of different age-classes. 

Several 1.000s of  LBBG can be present up to mid September. In these groups the races graellsii and  intermedius are both present, but hard evidence to categorize adults to certain races is very difficult, and classification is often only possible when birds are banded / ringed. From mid-October numbers of LBBG decrease rapidly. 

moult in the primaries

August is normally a month when primary moult is easy to detect, even on adult birds with folded wings. A small survey of adults in the Maasvlakte colony did not point to a significant primary moult delay in adults with begging youngs in the colony. Individual variation in adult LBBG within a certain colony seems to be at least as large as primary moult variation between colonies of Dutch intergrades and presumed intermedius from Scandinavia. In the table below, remaining old primaries are plotted for mixed groups graellsii, Dutch intergrades and intermedius at the Maasvlakte in the second half of August. Probably, most birds originate from the nearby colony and are Dutch intergrades, but large groups of Scandinavian birds arrive from mid-August onwards. 

Intermedius LBBG from Denmark and southern Norway (intermedius) probably arrive later on the breeding grounds in spring, compared to Dutch intergrades. Subsequently, it may be expected that egg-laying date is later in these populations. It remains to be seen whether or not they show delay in primary moult stage. Such surveys should only involve ringed (certain origin) adults. At this moment, such surveys are impossible; comparable analyses are unreliable by lack of sufficient ring observations.
So far, the hypothesis that intermedius have delay in primary moult could not be verified, but if it's true, the average remaining old primary figure in the table is overestimated (if read as applying to Dutch intergrades) by the input of intermedius arriving in August and later on. On the other hand, it is expected that Dutch intergrades with low moult score (arrested or suspended moult) are keen to leave the Netherlands as soon as possible, leaving a group of adults with relatively high moult score. At this moment, the figures in the table can be used as indication for "what average moult stage can be expected in the second half of August in LBBGs of various origins, present in the Netherlands".

tables