SPAANS MJ & SPAANS, AL (1975) Some data on the breeding biology of the Herring Gull Larus argentatus on the Dutch Frisian Island of Terschelling. LIMOSA 48 (1): 1-39.

European Herring Gull Larus argentatus 1. During four consecutive years (1966-1969) some aspects of the breeding biology of the Herring Gull have been studied on the Dutch Frisian Island of Terschelling (Fig. 1). 2. The first eggs were laid in the last days of April or in the first days of May, the bulk followed in the course of May (Fig. 2). The eggs laid after the first ten-day period of June probably concern repeat clutches. In 1967, 1968, and 1969, the mean dates of clutch commencement in all nests in which eggs were laid were 20.5 May ? s.d. 9.6 days, 18.9 May ? 7.1 days, and 19.0 May ? 8.3 days respectively, which is rather late in comparison with other countries in north-western Europe (Table 1). There is a positive relationship between the dates of clutch commencement in different years for nests in which eggs were found in two or more years (Fig. 3). 3. In 1967-1969 the mean nest density in area B (60,562 m2 ) amounted to 30.5 nests per ha. 4. In 1967-1969 the mean clutch size amounted to 2.74 eggs (Table 2). In 1966-1969 the mean clutch size was more or less constant till the 20th of May (peak of laying), but declined for clutches started later in the season (Table 3). 5. In 1969 the length and breadth of most eggs laid in the study area have been measured. The volume of the eggs was calculated using the formula V=kLB2 ? Experimentally, for k a mean value of 0.5035 was found. The mean volume of the c-eggs (83.74 cc) was much smaller than that of the a- (89.39 cc) and b-eggs (89.15 cc). The mean egg volumes of c/2 were 0.65-2.06 cc smaller than those of c/3 of the same laying dates. The mean volume of late c/l was even 5.99 cc smaller than that of late c/3 (Table 4). In c/3, the volumes of the earliest eggs were greatest, those of the latest eggs smallest, the differences being largest for the c-eggs (8.4%) and smallest for the a-eggs (3.0%) (Table 4). 6. In 1967-1969, on an average 76.5% of all eggs hatched. The main causes of egg loss were infertility and/or embryonic death and, to a lesser extent, predation. In 1967-1969 hatching succes amounted on an average to 78.8% for the eggs in c/3, 69.6% for the eggs in c/2, and 19.0% for those in cll (Table 5). The lower hatching success of the eggs in c/2 and c/l mainly resulted from an increase in the number of eggs that disappeared. In 1966-1969 hatching success decreased in the course of the season, both for c/3 and c/2, which mainly resulted from an increase in the percentages of infertile eggs and/or of eggs of which the embryo had died, and of eggs that disappeared (Table 6). There was hardly any difference in hatching success in relation to the sequence of laying (Table 7). The data might indicate that the b- and c-eggs have a higher chance to roll outside the nest than the a-eggs. Likewise the b- and c-eggs seem to suffer a somewhat higher mortality during hatching than the a-eggs. 7. In 1967-1969, on an average at least 58.2% of the chicks are known to have reached the age of 6 weeks, i:e. fledging age (Table 8), while this is assumed for at least another 10.0% of the young (mean of 1967 and 1968). The latter were seen alive in their fifth or sixth week of life, but not observed after that age. In 1966 (but not in 1967-1969) original b/3 suffered a significant higher chick mortality than b/2 and b/1, probably as a result of the cold and wet summer of 1966 (Table 9). Chick mortality was highest in the first week of life (Table 10). For a large number of chicks known to have died, the cause of death was unknown. Predation and cannibalism formed only a small part of the total chick mortality (18% of all deaths noticed). In 1967-1969 the number of chicks known to have fledged was not influenced by the original clutch and brood sizes (Table 11). In 1967-1969 fledging success decreased for all clutch sizes from early to late young (Table 12). Chicks hatched from c-eggs survived less well during the chick stage than chicks hatched from a- and b-eggs (Table 13). This mainly results from the higher mortality of the chicks from c-eggs during the first week of life, which is due to the smaller size of these eggs and the fact that they hatch some time after the a- and b-eggs (Table 14). 8. In 1967-1969 overall reproductive success amounted on an average to at least 1.21 young per pair, but it is assumed to have amounted to at least 1.35 young per pair (Table 15). When we disregard the year 1969, because of less intensive searching for chicks in their fifth or sixth week of life, overall reproductive success amounted to 1.41 young per pair (mean of 1967 and 1968). In? 1966-1969 overall reproductive success decreased from at least 1.68 young per pair for the earliest gulls to at least 0.41 young per pair for the latest gulls (Table 16). 9. It is argued that the numbers of young are regulated in the late summer and early autumn, when the gull population is at its maximum and the natural feeding possibilities are declining rapidly, and that the only manner to control the numbers of gulls effectively is to reduce the foraging possibilities on refuse dumps and other favourable feeding sites for gulls created directly or indirectly by man.

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