Part 7 Mediterranean Gull, White-winged Black Tern and Gull-billed Tern This series now turns to three species of the Laridae. The sources of bias in the data presented here and other points relevant to the whole series were discussed in the general introduction (Brit. Birds, 62: 169-174). Mediterranean Gull Larus melanocephalus When The Handbook was published (1938-41), Mediterranean Gulls were still extreme rarities in Britain (and unknown in Ireland): indeed, only four records are now accepted up to that time, six others having been discarded among the Hastings Rarities (Nicholson and FergusonLees 1962). It is exceedingly difficult to determine the number recorded in Britain and Ireland during 1958-67, since one individual may be seen at several localities or several individuals at one locality and birds at the same place in several years may or may not be the same individuals) reappearing. Counting one that reappears in several years as a new record on each occasion, but attempting to avoid duplication of wanderers, one can give a conservative estimate of 285 records in the ten years. That this is an arbitrary figure is demonstrated, however, by my calculation of 97 records during 1958-62 compared with 117 estimated from the same raw data by Bourne (1970b). Ages were not published for 32% of the records, but the remainder were 138 adults and 5 6 immatures or sub-adults. This proportion (approximately 5:2) may not reflect the true situation, however, for adults are easier to detect and identify than immatures and, as shown in