adult: June

Last update of this page: 07-12-03 17:33

At the Maasvlakte colonies in the Netherlands (see map), eggs are hatched by the first week of June and many nests contain three nestlings (breeding season 2001 and 2002). Pulli can be found everywhere. First data in 2001 estimate the colonies to be expanded by over 20%, now approximately 40,000 nests at the Europoort location alone (greater Rotterdam district). New colonies arise at plots which are still not occupied by industrial companies. 

In this picture you see female E832, ringed at the nest at Maasvlakte July 1995, now in her 6th summer and raising three pulli. Normally the elder adults occupy the sites used last year(s), as these birds know safety, feeding routes and places, etc. New colonies seem to be occupied by younger adults, as some ring-data suggest. Older LBBGs disturbed from 'ancient' colonies seem to move less far from the places they were forced to abandon. It's interesting to find out whether new colonies abroad are started by relatively young adults.

Moult:

Raising pulli is very time consuming and all moult in the wing-coverts is suspended in adults.
In some individuals, head feather moult is started mid-June and only the odd individuals moult coverts at a low pace. 

Counting back from the results of surveys done in August 2001, a small proportion of adult LBBG must have shed inner primaries by June. Last years, we did not collect sufficient material to estimate primary moult scores for this age class in June, so a small survey on 200 adult birds was done. We scored preening and bathing adults from the near colony (several 1.000's breeding pairs). The first estimation was done on arriving birds, checking the age while bathing and check primary moult score again at departure. The table shows the results: 75% of local (!) adult LBBG have still all the primaries old (p1-p10 old) and a small minority of about 20% has started moult with dropping p1.

Tables & Graph

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries throughout the year (arrested moult excluded). Sample size is 6.171 birds.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries for June (arrested moult excluded). Sample size for this graph is 339 ringed birds + 401 unringed birds.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls throughout the year. Longest new primary is classified "fully grown". Sample size is 2.969 birds. Birds scored on NEW primaries (arrested moult excluded).

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Scoring on longest new primary, which is classified "fully grown" for June (arrested moult excluded). Sample size for this graph is 10 ringed birds.

The earliest date for birds with missing primaries is 30 April 2006, which is not the earliest day in our complete sample. We scored 98 Lesser Black-backed Gulls prior to that date, some already in February. As can be expected, none of these was in primary moult, and this group was excluded in our further analysis as it will only result in a misfit for the onset of moult, using linear regression statistics.

The last scoring days were in November, with a larger sample (133 adults) on 04 November 2010. From that date we have the last birds with retaining old primaries (13 birds with 1, 2 or even 3 old primaries; 120 birds with no old primaries left). From other days in November, only 10 birds were scored (all ringed adults). They all had no old primaries left.

The first tine a new primary (P1) was visible, was on 08 May 2006. Prior to this date, birds were scored missing inner primaries, but the very tip was still not visible (and therefore not scored). As soon as the tip of P1 was visible, this was automatically scored "fully grown" For the other primaries to be scored fully grown, they had to exceed the previous primary in length (so P2 fully grown, only from when it is longer than P1).

LINEAR REGRESSION

Onset of primary moult can best be illustrated by a sinusoid for the first few weeks and last weeks, but there appears to be linear dependency over much of the central moult period.

LINEAR REGRESSION FOR OLD REMAINING PRIMARIES

Complete sample: Linear regression (n = 6171, R2= 0.820, sign: P<0,000)
PMS = -0,381* WEEK + 17,967

Ringed adults: Linear regression (n = 2069, R2= 0.800, sign: P<0,000)
PMS = -0,325* WEEK + 16,270

Unringed adult: Linear regression (n = 4102, R2= 0.779, sign: P<0,000)
PMS = -0,513* WEEK + 23,031

LINEAR REGRESSION FOR NEW FULLY GROWN PRIMARIES

Complete sample: Linear regression (n = 2969, R2= 0.930, sign: P<0,000)
PMS = 0,302* WEEK - 5,933

Ringed adults: Linear regression (n = 1209, R2= 0.888, sign: P<0,000)
PMS = 0,252* WEEK - 4,747

Unringed adult: Linear regression (n = 1760, R2= 0.833, sign: P<0,000)
PMS = 0,442* WEEK - 11,906