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DARLING (1938) suggested that synchrony of laying in colonial birds is stimulated by interaction between the birds, and the larger the colony, the greater the stimulus. Hence the laying in larger colonies will be more synchronized than that in small ones. In Darling's words, "the degree of breeding synchrony by social stimulation may be densityrelated" (Darling 1938: 53). Since then various authors have examined these ideas for different species of birds. Some students found such density correlated onset of laying to occur (Coulson and White 1960 in the Black-legged Kittiwake, Rissa tridactyla; Horn 1970 in the Brewer's Blackbird, Euphagus cyanocephalus; Nelson 1967 in the Gannet, Morus bassana), while others found that they could not confirm the phenomenon in various species of gulls, Larus [spp.] (MacRoberts and MacRoberts 1972 in L. argentatus and L. fuscus; Vermeer 1963 in L. glaucescens; Weidmann 1956 in L. ridibundus). In addition to these observations, Ashmole (1963), Hailman (1964), and Harris (1970) suggested that "display interaction" or "social stimulation"' cause breeding synchrony in some "'non-annual breeders" (the Sooty Tern, Sterna fuscata, and the Swallow-tailed Gull, Creagrus furcatus, respectively). Darling (1938) suggested that synchronous laying decreased predation, and Brown (1967), Parsons (1971), Patterson (1965), and Kruuk (1964) proved that Herring, Lesser Black-backed, and Black-headed Gulls (Larus argentatus, L. fuscus, and L. ridibundus respectively) that breed at the peak of the breeding season had better breeding success than individuals that breed earlier or later than the peak. Patterson (1965) also showed that Black-headed Gulls that breed outside or on the edge of a colony were more vulnerable to predation and thus less successful than birds in the center. "Edge effect" (i.e. lower breeding success on the edge of a colony) was reported for the Gannet (Nelson 1967) and the kittiwake (Coulson and White 1956). Horn (1968) found that "clumped nesting in Brewer's Blackbird improves foraging efficiency and predation avoidance only when the colony is built in a large expanse of nesting habitat, surrounded by abundant, but patchily distributed, food." All the species mentioned breed in dense colonies, where the distance between the closest nest is rarely more than a few meters, but there is no reason why territorial species should not synchronize breeding in order to benefit in the way that some colonial birds do. For example

These observations show a seasonal variation in the feeding activity of a group of Kittiwakes, though dependent on a sewage outlet in spring; the birds moved out to sea for food at the height of the breeding season. The reasons for the change are discussed.

The uropygial gland fat from the kittiwake is a complex mixture of ester waxes. As acidic components mainly n-octanoic acid and 2-methyl-substituted fatty acids are found. Besides 2,6-, 2,3 and 2,10-dimethyl-substituted fatty acids could be detected. Among the alcoholic components n-, 2-methyl-, 2,6-, 2,8-, 2,10- and 2,12-dimethyl-substituted primary alkanols were observed. Chemotaxonomically the kittiwake should be placed between the puffin and the limicoles.

Clutch size of the kittiwake is influenced by the previous breeding experience of the female and also by the time of laying. Both of these factors play a part in determining the clutch size and the correlation between breeding experience and clutch size is not solely the result of older birds breeding earlier. The clutches laid by females breeding for the first time are, on average, smaller than those laid at the same time by more experienced breeders and are less influenced by the time of breeding. No female breeding for the first time has been found to lay three eggs. There is a correlation between the time of the onset of breeding in kittiwake colonies and the average clutch size. It is suggested that a progressive decrease in the clutch size as the breeding season progresses may be widespread in both passerines and non-passerines and probably occurs only in species which do not normally have two broods in a single breeding season.

The calls and displays of the swallow-tailed gull (Creagrus furcatus) are described and fitted into the pattern of larid displays; some ecological aspects of reproductive behaviour are briefly described. 1. The striking vocabulary includes a scream, snore, rattle and whistle. There is a 'long call' apparently homologous with the generalised gull long call and a special call associated with downward choking. 2. Swallow-tailed gulls breed in relatively small groups, the reproductive activities of the population of any island being highly unsynchronised, though those of the group significantly more so. The 'local' synchronisation is probably socially facilitated. 3. Fighting is rare, but includes a twisting movement reminiscent of the kittiwake's (Rissa tridactyla) cliff-adapted method. 4. Threat gaping is shown. 5. The oblique postures may be separated into (a) the retracted oblique (probably not equivalent to the generalised forward oblique of some other gulls) (b) a low oblique (probably homologous with the generalised oblique of other gulls). 6. A downward jerking movement may be a form of long calling and swallow-tails show a backward head jerk in closequarter territorial encounters. 7. An aggressive upright may be distinguished from an anxiety upright. 8. Upward choking (as in the kittiwake, but not in other gulls) and downward choking occur; both forms are hostile displays on the nest site. 9. The butterfly flight is probably strictly an intra-pair display and is associated with snore-scream calls. It may be related to the soar-and-swoop of the black-headed gull (Larus ridibundus), which is probably ritualised pursuit flight. 10. In the swallow-tail foot-looking is commonly associated with agonistic behaviour. 11. Courtship feeding of female by male is a regular part of pre-laying pair behaviour as in other larids. 12. Pre-copulatory behaviour is accompanied by head tossing. 13. Nest building is accompanied by long bouts of nest site calling. The nest is architecturally functionless. 14. The single egg (incubation period 35 days and incubation stint about 24 hrs) produces a highly cryptic chick. 15. Egg shells are not systematically removed. 16. The chick may accompany its parents some distance from the site in the first 48 hours after hatching. 17. Squids form a large part of its diet. The position of the swallow-tailed gull with respect to other larids is tabularised and discussed.

The size and shape of kittiwake eggs are influenced by several factors, in particular by the position of the egg in the laying sequence, the breeding experience of the female and a possible inherited effect which tends to make the same female lay similar eggs in successive years. The adult kittiwake becomes larger towards the north of its range (Bergman's Rule) and probably as a consequence of this, the eggs also tend to be larger towards the north. In general, as female kittiwakes become older, their eggs become shorter and broader while the shape index (breadth × 100/length) and volume also tend to increase. These changes become less marked as the birds become older but there is no indication of the breadth and egg volume becoming smaller in older birds although very few birds over eight years of breeding age were studied. It is shown that several Laridae laying clutches of three eggs lay a final egg which differs markedly from the first two in both shape and size. In the Kittiwake, this difference (as nieasured by the variance) increases with age but no such difference occurs in clutches of two eggs. The use of the egg characteristics as a means of determining the age composition of breeding females in populations or colonies is discussed and applied to two colonies of kittiwakes where the age composition was unknown. It is suggested that, according to the species and the clutch size, the shape index and the variance of the breadth within a clutch are probably the most useful characters for such a method.

The supraorbital salt glands were surgically removed from five nestling kittiwakes, Rissa tridactyla, which, with four sham-operated birds, were reared in individual cages for 29 days on a diet of weighed fish. The daily cloacal excreta were weighed and analyzed for Na+, K+, Cl−, and water. Salt gland removal did not greatly affect cloacal ion excretion. In both groups cloacal fluid contained 30% of the ingested water and 20–50% of the ingested ions. The cloacal fluid of control birds was always hypotonic to the food, that of glandless birds hypertonic. In all samples the solid excreta (mainly uric acid) comprised 10% of the weight, but contained 25–50% of the cloacally excreted Na+ and Cl− and up to 60% of the K+. Fecal material might account for 10% of the ingested NaCl. Since cloacal excreta contained only one-half of the ingested ion load, extrarenal salt excretion may still have existed in the kittiwakes lacking supraorbital salt glands. These birds did not secrete after acute salt loading, but the plastic cage liners of these birds had nearly as much NaCl on them as did those of control birds, suggesting secretion from the head region. The tears were approximately isotonic to the plasma. Salt gland removal did not affect adrenal, lacrymal, or Hardenian glands, kidney, or body weight, but glandless birds had slightly larger hearts.

SYNTHETIC organochlorine derivatives have been found in a variety of seabirds from the eastern North Pacific1 and even the Antarctic2, and considerable amounts of polychlorinated biphenyls (PCBs) in particular have been found in birds from the Baltic3, Dutch waters4 and the livers of starving common guillemots, Uria aalge, washed ashore around the Irish Sea in the autumn of 19695. But comparatively few estimates of the amounts present in normal seabirds from the open North Atlantic have yet been published. We are reporting elsewhere6 the results of the investigation of over 100 birds from a representative range of species occurring between Scotland and the Arctic. In general they were all found to contain mainly PCBs and pp′-DDE, the former in greater concentration in every bird examined, in marked contrast to the North Pacific, where DDE levels are normally higher1. The ratio of PCB to DDE was usually between two and ten in all species with the exception of the kittiwake, Rissa tridactyla, where the ratio was always higher. It is difficult to measure DDE in the presence of relatively high amounts of PCBs but in some of the kittiwakes this ratio was at least 60. This suggests that kittiwakes have a food source relatively high in PCBs compared with the other species or, less likely, that they metabolize or excrete DDE more rapidly than the other species.

Eimeria rissae n.sp. was obtained from the small intestine of a kittiwake which had been sent to us as a representative of a number of sick and dying kittiwakes from the coast of Wales.

YOUR reviewer, in dealing with Mr. R. J. Ussher's work on the birds of Ireland (see NATURE, November 29, 1900, pp. 101 and 102), had his attention particularly drawn to two statements concerning the eggs of the guillemot. In the first of these, which occurs on p. 364 of his book, Mr. Ussher puts forward the suggestion that “the beautiful varieties of colouring must help each bird to distinguish her egg from others lying near until they all become stained and soiled” (the italics are mine). This is certainly a very pretty hypothesis; but is not the earlier part contradicted by the part I have italicised? It is certainly indirectly contradicted by a statement on p. 365, where Mr. Ussher records his belief that when the eggs of the guillemot are found, as they sometimes are, in the nests of cormorants and kittiwakes, “the owners of the nest incubate the mixed clutches, and not the guillemots, for I have noticed (he writes) a kittiwake chase away one of the latter from its nest.”

IN many bird colonies there is a surplus of adult birds in or about the colony which fail to breed1,2. In several species, the failure to breed can be attributed to the inability to obtain a suitable nest-site within the colony limits, and this results in competition for the limited number of available sites. Experiments on the kittiwake Rissa tridactyla have shown that some of these non-breeding adults will breed if suitable unoccupied ledges are made available in the centre of the colony, but similar sites remain unused on the periphery (Coulson, in preparation). This and other observations indicate that the competition among males, which are responsible for nest-site selection, is more intense in the colony centre and investigations were made to determine whether the birds nesting centrally in the colony differed in quality from those breeding near the edge.