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.     

DISEASE IN A COLONY OF BLACK-HEADED GULLS LARUS RIDIBUNDUS

An account is given of the disease factors encountered in a colony of Black-headed Gulls in the breeding season 1956.     

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.     

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.     

THE FOOD OF SOME LARUS GULLS

The food of L. argentatus on Skomer and Skokholm Islands (Wales) was studied by the analysis of stomach contents, pellets and direct observation. L. argentatus is mainly a scavenger and exists for the most part on fish-waste and garbage discarded by man. At Ncwborough Warren argentatus feeds more on arable land and on the shore than do the Skokholm and Skomer birds.
Although argentatus frequently eats limpets Patella spp., there is little competition for food between them and Oystercatchers Haematopus ostralegus because the two species eat different sized limpets.
A summary of previous studies into the food of argentatus is given.
L. marinus is mainly a predator feeding on the young of other gulls, sea-birds and Rabbits but it also eats carrion, offal and fish-waste when available and during the winter. The numbers of Manx Shearwaters Procellaria puffinus killed on Skokholm and Skomer are discussed.
L. fuscus obtains most of its food on the shore and on arable land. There appears to be little competition, at least in the areas studied, between .fuscus and argentatus for food.     

BREEDING SUCCESS AND POPULATION GROWTH IN A COLONY OF HERRING AND LESSER BLACK-BACKED GULLS LARUS ARGENTATUS AND L. FUSCUS

This paper describes the results of investigations into the factors affecting breeding success of the Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus, in the large colony on Walney Island, northwest Lancashire, between 1962 and 1965. These investigations were concerned with the incubation period, and the first ten days after hatching. The survival of chicks to ten days is 67% in Herring Gulls, and 56% in Lesser Black-backs. Most of these losses occur in the period just after hatching and are due to “cannibalism” by other gulls. This form of predation does not appear to be masking any effects from starvation or disease. The following factors contribute to egg or chick mortality:breeding too late (and, to a much smaller extent, too early); breeding in the open, as opposed to amongst cover; the facts that eggs in small clutches have a lower hatching success than those in large ones and that Herring Gull (but not Lesser Black-back) chicks in small broods are less likely to survive to ten days than are those in large broods. Chick mortality after the first ten days is not certainly known. About 30% of the eggs laid gave rise to fledged young— or about one fledged chick per pair. In the Herring Gull, the average clutch size (2.56) is lower than that of the Lesser Black-back (2.76). Both species show a seasonal decline in clutch size—this occurs earlier in the Herring Gulls than in the Lesser Black-backs. The Walney population, which stood at about 700 pairs in 1950, had reached 12,000 in 1957, and is at present about 18–19,000 pairs. It is suggested that this increase may be linked to the greater availability, or exploitation, of human garbage in the Morecambe Bay area. The population explosion between 1950 and 1957 must have been partly due to massive immigration and could not have come about through natural increase alone. The possible influences of the gulls' behaviour on the population growth are discussed. There is no evidence of any “shock disease”, although the Walney colony is very crowded. “Cannibalism” is regarded, not as evidence of a failing food supply, but as an extension of the normal hunting behaviour of these omnivorous gulls; it will be an economical means of obtaining food only in a large, dense colony, such as Walney. It may be offset by increased breeding efficiency due to social factors.     

THE MOULT OF REMIGES AND RECTRICES IN GREAT BLACK-BACKED GULLS LARUS MARZNUS AND GLAUCOUS GULLS L. HYPERBOREUS IN ICELAND

The moult of primaries, secondaries, and rectrices in two closely-related gulls, the Great Black-backed Gull Larus mavinus and the Glaucous Gull L. hyperboreus, was studied in Iceland. Both gulls moult their primaries in an extremely regular sequence, starting with the 1st (innermost) and ending with the 10th (oiltermost) feather. Usually two, less often one or three, primaries are growing per wing during the primary moult, which lasts for about six or seven months. Growlng primaries were estimated to lengthen on the average by 8.7 mm per day in marinus and 7.8 mm per day in hyperboreus. The secondaries, usually 24 in number, are shed in two moult waves, one starting with the innermost feather soon after the start of the primary moult and then progressing slowly outwards, the other beginning with the outermost secondary after the primary moult is about half completed and then progressing rapidly inwards. The moult is completed just before the end of the primary moult as the two moult waves meet at about the 16th secondary. There are no marked differences between the two gulls in the moult of secondaries. The moult of rectrices shows large variations in both species, some feathers being much more irregular than others in their time of shedding. In both species, indications of an obscured centrifugal pattern of replacement are seen, although the 5th (next to the outermost) rectrix is usually the last one to be shed. Significant differences were observed between the two species in the degree of regularity of shedding of some feathers and in the average position in the moulting sequence of others. The moult of rectrices starts soon after the moult of primaries is half completed. The feathers are then shed in rapid succession, and the moult is completed some time before the end of the primary moult. The need for good powers of flight at all times is undoubtedly the reason for the protracted primary moult. This in turn causes the moult to start early, in adults sotnetimes before the eggs are laid; immatures moult even earlier than this. The rectrix moult and the main part of the secondary moult do not begin in adults until the young have fledged, but then progress very rapidly. Presumably, the loss of some of these feathers would impair the flying ability to an extent sufficient to make it difficult for the gulls to care for their young, while the rapid moult is necessary in order for the replacement of these feathers to be completed by the time the primary moult is over.     

KLEPTOPARASITISM OF SANDWICH TERNS STERNA SANDVICENSIS BY BLACK-HEADED GULLS LARUS RIDIBUNDUS

The parasitic behaviour of Black-headed Gulls in a mixed colony of terns and gulls at the Sands of Forvie on the northeast coast of Scotland is described in some detail. Food-stealing occurred with varying frequency throughout the breeding season. Less than 6% of the terns were attacked during incubation and when their chicks were a few days old, but up to 29% were attacked thereafter. At the same time the percentage of successful attacks rose from 1% or less to 6.5%.
Food items brought back to the ternery were predominantly sandeels, clupeids and gadoids. Only a small proportion of terns carrying fish shorter than 7 cm were attacked whether they were sandeels, clupeids or gadoids.
Robbing success was higher with clupeids and gadoids than with sandeels.
The effect of the Black-headed Gulls' kleptoparasitism on the Sandwich Terns' breeding success is thought to have been negligible during incubation and early chick-life, but might have influenced fledging weight and ultimately post-fledging survival.     

MIGRATION AND DISPERSAL OF AUDOUIN'S GULL LARUS AUDOUINII FROM THE EBRO DELTA COLONY

Oro, D. & Martinez, A. 1994. Migration and dispersal of Audouin's Gull Larus audouinii from the Ebro Delta colony. Ostrich 65:225-230.

Migratory paths and dispersal patterns of Audouin's Gulls (Larus audouinii) born in the Ebro Delta have been studied, as well as their degree of philopatry. The gulls always disperse southwards from the colony, and only the juveniles seem to disperse in other directions. Dispersal patterns of 2y, 3y and older gulls are similar, wintering mainly on Iberian Mediterranean and Moroccan Atlantic coasts. Juveniles migrate before the rest of the age classes and they winter south of older gulls, in the Senegambia region. However, conclusions on Audouin's Gulls movements are difficult to draw, owing to the large differences in resighting and recovery efforts. Data presented also suggest that Audouin's Gulls do not appear to breed before the age of four years.     

INTRASPECIFIC PREDATION AND COLONIAL BREEDING IN LESSER BLACK-BACKED GULLS LARUS FUSCUS

The breeding biology of Lesser Black-backed Gulls was studied on Skokholm Island, Pembrokeshire, where the number of breeding pairs has been increasing at about 20% per annum since 1963. Laying was found to be synchronous within small groups. Clutch size and breeding success showed seasonal declines over the spread of breeding. The loss mainly of eggs, but to a lesser extent of chicks also, caused this overall decline in success. Hide observations indicated that the bulk of these losses arose through predation by nesting adult Lesser Black-backed Gulls on their nearest neighbours. Not infrequently the protagonist had lost its own clutch shortly before turning predator, and such a chain-sequence should lead to a steady build-up of aggressive failed breeders and so account for the observed seasonal increase in egg loss. Attentiveness to the clutch decreased with season but this was unlikely to have been important in precipitating the predation. Intermediate plant cover was associated with highest nest density and also with highest chick survival. In addition, nest density was directly correlated with chick survival. Whether plant cover and nest density separately affect chick production remains unresolved. Nevertheless, the increasing nest densities in this colony caused by the growth of the gull population are thought to be responsible for the widespread intraspecific predation, the intensity of which is probably a new feature of the gulls' breeding behaviour. The implications of this 'internal' predation for laying synchrony and aggregated nesting are discussed; these two factors of the breeding pattern probably evolved largely to combat 'external' predators. Not only are they no protection against inter-neighbour predation but appear to facilitate it. It remains to be seen whether this kind of predation will significantly affect breeding patterns with further increases in nest density.     

FACTORS AFFECTING THE EGG SIZE OF RED-BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

The factors influencing the egg size of the Red-billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two- and three-egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one-, two- and three-egg clutches declined significantly as the season progressed. The size of eggs from single-egg clutches tended to be smaller than eggs from two-egg clutches laid at the same time. There were correlations between the proportions of one-egg and of three-egg clutches being laid at a given period and the mean egg volume of two-egg clutches. When the mean egg volume of two-egg clutches increased there was a corresponding increase in the proportion of two- and three-egg clutches laid. When the mean egg volume of two-egg clutches decreased there was an increase in the proportion of single-egg clutches laid. The egg size of the Red-billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time. Two-year-old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females. It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.