COLONY AND HABITAT SELECTION OF SIX KELP GULL LARUS DOMINICANUS COLONIES IN SOUTH AFRICA

Kelp Gulls nested in six colonies on rocky islands, sand and rock cliffs, sand dunes and on a sand, gravel island in a salt lake. The colony sites selected were different from the surrounding areas and were generally inaccessible to ground predators. Kelp Gulls nested in a wide variety of habitats. Within colonies, the gulls did not nest randomly with respect to habitats but preferred to nest on flat, stable areas with some cover (either rocks or vegetation). They generally avoided heavily vegetated areas, areas devoid of vegetation and areas with steep slopes. Their specific habitat choices relate to predation and cannibalism pressures. Intermediate cover provides adequate protection for chicks while allowing for increased visibility and open escape routes for parents. Nearest neighbour distances were similar among colonies and habitats, indicating strong social attraction.     

TIMING AND SPACING OF BROODS IN THE BLACK-HEADED GULL LARUS RIDIBUNDUS

The nests of the Black-headed Gull Larus ridibundus are closely aggregated into dense colonies and their use synchronized, these two phenomena together tending to produce a maximal clumping effect. Within such a colony however, nests were found to be spaced out to produce a non-random uniform distribution. The commonest distance between neighbouring nests was found to be about one metre, in contrast to related species. This study was concerned with two aspects of this distribution pattern; its survival value and its behavioural causation.
It was found that pairs nesting just outside the colony had a much lower breeding success than those nesting in the colony and that nests on the colony fringe had a slightly lower success than those in the centre. Pairs laying during the peak laying period had a higher breeding success than pairs laying either earlier or later in the season. Since by far the most important mortality agent was predation, it seems likely that both clustering and synchronization of nesting function as antipredator systems and arguments in favour of this are discussed.
Variations in nest-spacing within the colony were not correlated with variations in breeding success.
In the causation of the spacing between nests, territorial aggression was demonstrated to be an effective dispersion mechanism and the way in which this mechanism works was investigated in detail.
This spacing mechanism was not sufficient by itself to explain the observed densities, which were higher than one would expect from the aggression alone; there was also some tendency for birds establishing a new nest-site to cluster close to others. The interaction between this, the territorial aggression of the residents and the subsequent avoidance responses of the settling birds, can explain the nest spacing pattern and probably also the observed densities.     

NESTING DENSITY AND BREEDING SUCCESS IN THE HERRING GULL LARUS ARGENTATUS

The relationship between nesting density and breeding success of Herring Gulls Larus argentatus was studied on the Isle of May, Scotland, in 1968. Herring Gulls nesting at the most common density started laying earlier in the season than those nesting at lower or higher densities. Therefore, although the overall spacing of nests was uniform, the nest density of birds laying later in the season progressively approached a random distribution. The onset of laying occurred in synchronized groups within the colony. Late-laid clutches were commonly situated on the periphery of the colony where the density of nests was lowest. When the laying period was divided into four time periods, in each period the tendancy was for birds nesting at the most common density to have the highest clutch-size, hatching and fledging success, and to rear the most chicks per pair to fledging. In addition, birds which spaced their nests most uniformly, presumably as a consequence of territorial behaviour, were the most successful parents.     

DIET OF HARTLAUB'S GULL LARUS HARTLAUBU AND THE KELP GULL L. DOMINICANUS IN THE SOUTHWESTERN CAPE PROVINCE,SOUTH AFRICA

Steele, W.K. 1992. Diet of Hartlaub's Gull Larus hartlaubii and the Kelp Gull L. dominicanus in the southwestern Cape Province, South Africa. Ostrich 63:68-82.

Hartlaub's Larus hartlaubii and Kelp Gulls L. dominicanus in the southwestern Cape feed on a wide range of prey species. On average, Kelp Gulls feed on larger-sized prey than do the smaller Hartlaub's Gulls. In an undisturbed environment the preferred foraging habitats of Hartlaub's and Kelp Gulls are rocky shores and open, sandy beaches (where sand mussels Donax serra are common) respectively. However, as a result of man's activities, several new foraging habitats have become available including croplands, fishing harbours and refuse dumps. The provision of supplementary food at these new foraging habitats is likely to be the cause of a recent population increase. Kelp Gull chicks are fed predominantly “natural” prey which indicates that the population increase may not be due to enhanced reproductive success, but to improved post-fledging survival of juveniles, which are known to aggregate at sites where supplementary food is provided.     

SPECIES ISOLATION BETWEEN THE HERRING GULL LARUS ARGENTATUS AND LESSER BUCK-BACKED GULL L. FUSCUS

There is a large, mixed colony of the two “ring” species, the Herring Gull Larus argentatus and Lesser Black-backed Gull L. fuscus, on Walney Island, northwest Lancashire. These birds are nesting at the very high density of one nest/40 square yards, or more, but although they defend their territories against both species indiscriminately, there is effectively no hybridization. This paper discusses the nature of the species isolation mechanism, and its function. Since the two species can produce fertile hybrids, the mechanism must be of an ecological/behavioural nature, rather than morphological incompatibility. It is shown that there are slight differences in breeding season and habitat, but these do not seem to be great enough to account for the high degree of isolation. It is likely that species isolation depends primarily on the female's choice of a mate. It is suggested that, as specific cues, she uses the differences in call-note tones, and the colour of the back (and perhaps also of the eye-ring), or both. Herring Gulls and Lesser Black-backs are adapted to slightly different niches. The overlap is so great, however, that any hybrid is unlikely to be at a disadvantage; but the overlap can only have arisen very recently, as a result of both species taking advantage of the increased availability of human refuse. It is possible that the isolation mechanism was evolved to cope with earlier conditions, when food was more limited, and the species' niches more sharply defined.     

DISTRIBUTION,POPULATION SIZE AND CONSERVATION OF HARTLAUB'S GULL LARUS HARTLAUBIL

Williams, A. J., Steele, W. K., Cooper, J. & Crawford, R. J. M. 1990. Distribution, population size and conservation of Hartlaub's Gull Lorus hurtlaubii. Ostrich 61: 66–76.

Hartlaub's Gull Larus hartlaubii is endemic to southern Africa, where it breeds between Swakopmund, Namibia and Dyer Island, southwestern Cape Province, South Africa. The species has been re breeding at 48 localities within this range. Between 1984 and 1989 an estimated 12000 pain brered at 31 localities. Twenty-eet percent of the population breeds at Robben Island off the Cape Peninsula, sQuth Africa. Hartlaub's Gull frequently has low breeding success and is considered endangered in Narmbia, where 12% of the poulation occurs. However, the population is increaslng around the urbanmd Cape Peninsula where HartLub's Gull has the potential to become a pest species.     

AGE, EGG-SIZE AND BREEDING SUCCESS IN THE HERRING GULL LARUS ARGENTATUS

The breeding biology of the Herring Gull Larus argentatus was studied in 1969, 1970 and 1972 on Skokholm, Pembrokeshire. The distribution of laying dates in three years is given; median laying date was always about 10 May. Laying dates of individual pairs were fairly constant between two years, but females up to nine years of age apparently advanced their laying date in successive years. In 1969 there was a decline in clutch-volume index during the season but this was not found in 1970 or 1972. Females laid clutches of similar volume in successive years, although the volume did vary with age, increasing up to 7 or 8 years of age and then declining. Hatching success was highest in the earliest of four periods in 1970 (69%) and lowest in the last (51%); in 1972 the corresponding figures were 71% and 63% and the seasonal trend was not significant. Chick mortality was higher in 1970 (69%) than in 1972 (59%); in 1970 there was significant seasonal variation in chick survival but in 1972 there was not. In 1970 chick survival was positively correlated with hatching weight (and hence egg-size) and chick survival was also related to growth which was itself correlated with hatching weight. There were no such correlations in 1972. Mean chick production in 1970 was 0.60 per pair and in 1972 0.70. In both years later breeders were less successful, but the seasonal range of mean success was less in 1972. In the discussion it is shown that some of these differences between years could be due to changes in the age structure of the population and it is suggested that the correlation between egg-size and chick survival in 1970 may be an artefact of the age structure.     

COLONY DESERTION AND REPRODUCTIVE SYNCHRONY OF BLACK-BILLED GULLS LARUS BULLERI

Black-billed Gulls breeding in large dense colonies on river beds in New Zealand were found to have a high rate of colony desertion early in the nesting cycle. The hypothesis is developed that individuals faced with possible desertion after they have begun to nest can reduce the likelihood of being deserted by delaying their nesting activities until a large number of others is also ready to nest. This interpretation is developed into a graphic model to show that colony desertion can thereby produce a strong selection pressure favouring highly synchronized nesting. This model successfully predicts onset synchrony, subcolony formation, occurrence of complete colony desertions mainly early in the season, occurrence of partial colony desertions, and the existence of nester non-nester conflict over whether or not to desert. Unstable food appears to be the most likely cause of colony desertion in Black-billed Gulls. Black-billed Gulls are similar, in varying degrees to Sandwich, Royal and White-fronted Terns. Convergent traits include tendencies to occupy colonies rapidly and synchronously, reduced site tenacity, and the use of open and often unstable nesting habitats. The links described here between desertion and synchrony may be relevant for some other species.     

THE EFFECTS OF DIURNAL AND TIDAL PERIODICITIES IN THE NUMBERS AND ACTIVITIES OF HERRING GULLS LARUS ARGENTATUS IN A COLONY

The average numbers of Herring Gulls Larus argentatus present in a breeding colony on Walney Island, Cumbria, were found to vary with the tidal cycle but to remain effectively constant with time of day through the breeding season. An activity survey, based on 50 Herring Gulls observed at half-hourly intervals during March and April 1973, showed that sleep and rest varied inversely with each other with sleep increasing to 50 per cent at midday. After a peak in the proportion of gulls asleep four hours before low tide, sleeping progressively decreased until low tide; seemingly a result of resident gulls waking and remaining more alert as others left the colony in search of food. Preening was constant throughout the day and tide cycle. Other behaviours (mostly courtship and agonistic behaviour associated with territory defence) increased slightly during low tide and were more common early and late in the day. Night observations of the gulls' activities showed that there was a peak of sleeping between midnight and 02.00 hours. It is suggested that Herring Gulls have a bimodal diel sleep pattern.     

ADAPTATIONS TO NESTING HABITAT IN THE REPRODUCTIVE BEHAVIOUR OF THE BLACK-BILLED GULL LARUS BULLERI

SUMMARY The nesting habitats adopted by most colonies of Black-billed Gulls Larus bulleri are river-beds that are subject to flooding. A number of respects in which the reproductive behaviour of Black-billed Gulls differs from that of at least most other gulls, such as Black-headed Gulls, can be viewed as adaptations, or byproducts of adaptations, to such nesting habitats:—

• (a) A different breeding site from the year before is often selected
• (b) The bulk of pair formation is accomplished before the gulls occupy their breeding sites; nesting territories are set up by mated pairs
• (c) In hostile encounters during the pair formation phase, site attachments are weak or transitory
• (d) In agonistic situations generally, attack thresholds appear to be relatively high, and fleeing thresholds relatively low
• (e) High intensity forms of“Choking” appear to be missing from the agonistic display repertoire
• (f) The growth of nest groups is rapid and orderly; nesting territories are small and the concentration of nests in the groups high
• (g) The time between occupation of the gullery site and the start of laying is short, and the synchrony of laying is high; this is probably related to the close proximity of the nests
• (h) The parents and young abandon the nest very soon after the egg hatch, and no “brood” nests are made
• (i) The young may develop locomotory powers more quickly than is the case in most other species; they can swim at an early age, and take to the water in tightly packed groups during alarms

The question of why these gulls should choose such vulnerable breeding sites is discussed.     

有关棕头鸥和遗鸥两近似种的分类与分布问题研究

本文根据近几年的野外观察结果并分析有关文献资料,系统地比较研究了棕头鸥和遗鸥两近似种的分类和分布现状,直接依据野外活体形态并对照系列标本对两种鸥的成、幼鸟和雏鸟的形态及野外临别特征给出了较精确的描述,澄清了这两种鸥以往在分类上的混淆有分布认识上的错位,同时从生态－动物、地理和行为学的角度对两种鸥的现有分布和今后分布态势的演进做了分析论证。     

THE PROCESS OF COLONY FORMATION AMONG HERRING GULLS LARUS ARGENTATUS NESTING IN NEW JERSEY

We examined the pattern of colony occupation and egg-laying in five colonies of Herring Gulls nesting in New Jersey, U.S.A. Colonies formed from epicentres located in sparse bushes. The number of epicentres related to the number of birds nesting in the colonies. Colonies of over 250 pairs had more than one epicentre, whereas those with under 250 pairs had only one epicentre. Epicentres were not always in the geographical centre of the colonies. New territory-hunting pairs filled in the epicentre areas, and then nested outside these areas. The egg-laying pattern followed the settling pattern, but was more synchronous than the settling pattern. There was greater synchrony of egg-laying within sub-areas of the colonies than in the colonies as a whole. Further, synchrony correlated with the number of nests in sub-areas.