Female body condition and brood sex ratio in Yellow-legged Gulls Larus cachinnans

In the Yellow-legged Gull Larus cachinnans , males are the larger sex, and show more reproductive variance than females. We predicted that the proportion of male chicks in a brood should increase with female body condition. We investigated brood sex ratio by using DNA markers taken from samples of hatchlings or dead embryos, and female body condition using plasma cholesterol concentration as a reliable indicator. The brood sex ratio of females in good condition was male biased and the sex ratio of females in poor condition was female biased. This relationship was also significant in those nests where all the eggs laid were sexed. Thus, manipulation of embryo mortality cannot explain the biases reported in this study, suggesting that the sex ratio of the eggs was biased prior to laying. These results confirm that sex-ratio manipulation in gulls operates under natural conditions, and supports earlier experimental findings.     

The Effect of Habitat Structure and Density of Nests on Territory Size and Territorial Behaviour in the Black-headed Gull (Larus ridibundus L.)

We studied the effects of vegetation structure and nest density on territory size and aggression in the black-headed gull (Larus ridibundus). Vegetation structure influenced the types of aggressive behaviour, but not territory size. The proportion of overt aggression (attacks, fights) was highest in barren areas or those with short, sparse vegetation, and lowest in plots covered with dense, tall grass. We explain this by decreased visibility of conspecifics in habitats isolated by vegetation. In such areas, total aggression did not decrease, because the low proportion of attacks and fights was offset by an increase of more moderate (“upright”) displays. Only when nest density was high did the vegetation reduce the frequency of agonistic behaviour — but then it comprised almost exclusively attacks and fights. As expected, greater nest density was related to reduced territory size and an increased proportion of overt aggression. A high proportion of attacks and fights occurred in the plot with the lowest nest density, where at the same time there were also numerous first-year breeders. We conclude that, when analyzing the effects of habitat conditions on aggressive behaviour, it is important to consider the structure of behaviour, and not only the total frequency of all types of agonistic interactions.     

Movement of Black-headed Gulls from Colonies in England and Wales

Despite deaths and low breeding success in an orchard regularly sprayed with DDT-based pesticides, local populations of Blackbirds and Song Thrushes appeared to maintain their numbers. The authors discuss the reasons why.     

Egg shell removal by the Black-headed Gull (Larus r. ridibundus L.) II. The effects of experience on the response to colour

WESTWARD VAGRANCY OF SIBERIAN PASSERINES, by C. W. Holt and Kevin Baker

BREEDING BIOLOGY OF THE WHINCHAT, by R. F. Ruttledge and David Scott     

Agonistic display and spacing in the black-headed gull, Larus ridibundus

Black-headed gulls nesting on islands in a marsh habitat had a higher breeding success than those nesting on the periphery of the colony. Island birds maintained close inter-nest distances whereas peripheral nests were spaced further apart as predation of eggs increased. These changes in spacing corresponded with changes in the response distances of agonistic displays. There was no change during incubation for island birds but peripheral birds displayed to intruders at much greater distances than during pair formation. Island birds were more likely to attack and display more frequently to intruders than peripheral birds. However, in one season the peripheral birds were more aggressive during pair formation because of an increase in intrusions by unpaired females.     

Seasonal and Diurnal Changes in Aggression and Territory Size in the Black-headed Gull (Larus ridibundus L.) on Islands in the Middle Reaches of the Vistula River

Seasonal and diurnal changes in territory size and frequency of aggressive interactions of various intensity were examined in colonies of the black-headed gull (Larus ridibundus) on river islands and ponds of central Poland. The highest frequency of the total aggression and the largest territories were recorded before the egg-laying stage, this being related to the establishing of territories, defence of the mate, and defence of nest material. Later in the season both aggression and territory size declined. Unlike the large species of gulls, black-headed gulls did not increase their aggression and territory size during hatching of the young. High-intensity aggressive behaviours (choking, attacking and fighting) were not so frequently displayed as low-intensity aggressive postures (upright, long call, forward) in each phase of the breeding cycle. Presumably this was because the former were more costly in terms of energy. Peak territorial activity occurred in the late afternoon. An increase in the frequency of intense aggressive behaviours (attack, fight) at that time was combined with an increased mobility of birds (departures to and arrivals from foraging sites). A reduced frequency of high-intensity aggressive displays in the period when the largest number of birds was present in the colony was likely to be adaptive.     

Habitat selection and breeding success in Yellow-legged Gulls Larus cachinnans

The habitat selection and breeding performance of Yellow-legged Gulls Larus cachinnans were studied in the Medes Islands colony, northeastern Spain, during 1995 and 1996. Of the three main habitats on the islands (shrubs, grass and bare areas), gulls first occupied those with the highest percentage of tall vegetation. Gulls tended to select nest sites with 20–75% cover despite great differences in the cover in the habitats and territories, suggesting that the presence of a suitable nest site may play a major role in the choice of breeding habitat. Nest-site tenacity did not influence the preferences of gulls at any level since the same pattern of choice was observed in an area subjected to annual culls (i.e. where most of the breeding pairs were culled annually and replaced by naive birds). In spite of great differences in the physical characteristics of the habitats, little difference was found in breeding performance of the gulls between habitats. Gulls nesting in the least preferred habitat (i.e. mainly bare) had smaller clutches than those nesting in the other two habitats, possibly as a result of their later seasonal laying. Despite the similar breeding success in different habitats, gulls did not seem to distribute according to the ideal free model reported for Herring Gulls Larus argentatus since the density in the preferred habitat (i.e. shrubs) was never higher than in the other two. We suggest that the habitat selection by Yellow-legged Gulls within the colony could follow an ideal despotic distribution.     

Should males come first? The relationship between offspring hatching order and sex in the black‐headed gull Larus ridibundus

In birds with hatching asynchrony and sexual size dimorphism, chicks hatched earlier and later in the laying sequence usually suffer different mortalities due to uneven abilities to compete for food, especially in poor years. If sexes differ in vulnerability to environmental conditions, e.g., by having different food requirements due to differential growth rates, mothers can increase fitness by allocating sex according to the laying order, producing less vulnerable sex later rather than early in the clutch. By analysing variation in primary sex ratio using a PCR-based DNA technique, we tested this prediction in black-headed gull Larus ridibundus chicks where males may be the less viable sex under adverse conditions. The overall primary sex ratio of the population did not depart from parity. However, first hatched chicks were more likely to be males whereas last hatched chicks were more likely to be females. Both egg volume and hatchling body mass decreased with laying order irrespective of sex. Time of breeding had no effect on offspring sex or hatchling sex ratios.     

The influence of age and sex on phagocyte chemiluminescence

The process of ageing is associated with increased susceptibility to infection. Phagocytes form the primary defence mechanism against infecting microorganisms, but the influence of ageing on phagocyte function remains controversial. In this study we have applied a microtitre plate phagocyte chemiluminescence (CL) assay suitable for clinical use to compare phagocyte oxidative metabolism in younger healthy subjects (age 20–60 years) and healthy older (60–70 years) subjects. Polymorphonuclear leukocytes (PMNL) and monocytes were stimulated using phorbol myristate acetate (PMA), serum opsonized zymosan (SOZ), and non-opsonized zymosan (ZYM) in the presence of both lucigenin and luminol. Monocytes showed a higher luminolenhanced CL response to PMA in males compared with females in the younger age group. No PMNL differences were observed between the sexes. Although no difference were found in relation to age when cells were stimulated with PMA and SOZ, significantly lower background (unstimulated) CL was obtained from PMNL with luminol. PMNL luminol-enhanced CL responses were also lower in response to ZYM. The findings suggest a reduced response of PMNL from older subjects to minimal stimulation. This could be related to abnormalities in the triggering of the respiratory burst or myeloperoxidase release due to ageing. The influence of age and sex should be taken into account in clinical studies of phagocyte CL.     

Collective Waves of Sleep in Gulls (Larus spp.)

How should animals sleep in groups? Because sleeping reduces the ability of an individual to detect potential threats, not all individuals should sleep at the same time. The obvious solution of taking turns to sleep is not documented in animal groups. Individuals can also organize their sleeping bouts independently of each other but this simple strategy can be dangerous if too many individuals happen to sleep at the same time. One solution to this problem is to monitor the behaviour of other group members and adjust sleeping bouts accordingly. For instance, as the number of sleeping individuals increases, companions may decide that it must be a safe time to sleep. However, when fewer group members are sleeping, an individual may benefit by curtailing sleep, given that it would be more vulnerable than vigilant group members should an attack occur. Such monitoring can therefore lead to contagious behaviour in the group, which can be detected in a group by collective waves of activities through time. Using spectral analysis, I investigated the proportion of sleeping birds in loafing gulls (Larus spp.) as a function of time over 2 yr and found that in many groups, the proportion of sleeping birds rises and decreases in a systematic and statistically significant fashion. These results add more weight to the now increasingly supported view that vigilance in general is a social phenomenon and suggest that adaptive behaviour at the level of the individual can lead to collective phenomena such as waves of sleep in animal groups.