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The reproductive cost hypothesis predicts that parents with low investment capacity (e.g., those in poor condition) should produce the larger sex (usually males) less often so as to minimize the risk of current reproductive failure and/or a decrease in their residual reproductive value. The study aimed to test whether offspring sex ratio bias in replacement clutches of the Mew Gull (MG;Larus canus) and Black-headed Gull (BHG;Chroicocephalus ridibundus) exposed to different food conditions was in agreement with predictions of the reproductive cost hypothesis. Food availability was experimentally modified before egg laying. Our experimental treatment affected hatching sex ratio, being also significant the effect of laying order and species identity. For both gull species, parents that were provided with supplemental food had more sons at hatching than unfed (control) pairs. This was particularly so if they were fed until production of a replacement clutch. This gradient in hatching sex ratio was much more pronounced in MG than in BHG. In both gull species, sons hatched more often from first laid (A) than from second (B) or third-laid egg (C). Five days after hatching, the sex ratio of surviving chicks was dependent mainly on the experimental treatment and egg sequence. Unlike the time of hatching, there was no important effect of species × treatment interaction. In both gull species, the brood sex-ratio was clearly male-biased in parents receiving supplemental feeding. However, differences between experimental groups were less extreme than at hatching. Chicks hatched from C-egg had much lower survival than chicks from the first two eggs (A- and B-egg), irrespective of sex, treatment, or species. Supplementary feeding was associated with improved chick survival for both gull species, although this effect was dependent on chick gender. Surprisingly, in the case of control broods (i.e., broods whose parents did not receive supplementary food), male chicks survived better than female chicks, but the opposite was true for broods in which parents were fed. There was an overall tendency for higher survival of BHG chicks than MG chicks. During the early-chick period, competition between siblings and the related differentiated mortality of chicks from A-, B-, and C-eggs, constituted a mechanism which shaped the brood sex ratio bias. In this context, differences in the sex ratio bias in eggs of different rank (A, B, C) at hatching were important. The results appear to fulfil the expectations of the reproductive cost hypothesis. For the species with greater sexual dimorphism (MG), a relationship between the condition of parents and the expected bias of sex ratio was visible only at hatching, which may indicate that this feature is not significant in shaping the sex ratio bias in MG.

Sex allocation theory predicts that parents should adjust their brood sex ratio to maximize fitness returns in relation to parental investment. Adaptive adjustment of sex ratio may be driven by differential costs of rearing sons and daughters or differential benefits of investing limited resources into offspring of different sex. In both cases, possible sex ratio bias should depend on parental condition. For sexually dimorphic birds with males larger than females, sons may be less likely to fledge since they are more vulnerable to food shortages or because they have impaired immunocompetence due to higher testosterone levels. Poor condition females should thus overproduce daughters to minimize possible reproductive failure. We manipulated the number of eggs laid and the amount of food available to laying females to induce differences in the condition in 2 gull species differing in sexual size dimorphism. In the Black-headed Gull (Chroicocephalus ridibundus), sexual size differences are marginal; but in the Mew Gull (Larus canus), males are 11% larger. In both species, females forced to lay an additional egg (presumed in worse condition) overproduced daughters, whereas females receiving supplemental food before laying (presumed improved condition) overproduced sons. This sex ratio skew was larger in Mew Gull, a species with larger size dimorphism. Chick immunocompetence at hatching was unrelated to sex, being higher in broods of fed mothers and lower for chicks hatched from last-laid eggs. Chick survival between hatching and day 5 post-hatch was positively related to their immunocompetence, but chicks from last-laid eggs and males of Mew Gull, the more dimorphic species, survived less well. Results indicate that costs of raising larger sex offspring coupled with parental condition shape brood sex ratio in populations studied. Adaptive brood sex ratio adjustment occurs mostly before egg laying and includes differential sex allocation in eggs depending on the probability of producing a fledged chick.

We conducted our study in the Common Tern colony (STH) located on an island in the middle Vistula River course, at the height of the city of Dęblin (km 393–394 of the waterway), in 2017. Our goal was to investigate some aspects of the biology and reproductive ecology of this species. Due to the fact that STH breeds both in single-species as well as in two- or multi-species colonies, in associations with Little Terns (Sternula albifrons), Black-Headed Gulls (Chroicocephalus ridibundus) (LAR) and/or Mew Gulls (Larus canus), we wanted to investigate whether the neighborhood of other species (in this case LAR) affected hatching success and chick survival in STH. Our results clearly show that the presence of breeding terns in the neighborhood of the LAR colony was not accidental and/or caused by the lack of space on the island and/or the possibility of nesting elsewhere. The height of nesting site, type of nesting habitat, clutch size, mean egg volume and mean egg mass of these STH pairs did not differ significantly from those that formed a single species colony, on the same island but several hundred meters away. However, STH nests in the neighborhood of the LAR colony were established much earlier and both the hatching success and chick survival of STH during the early-chick stage were twice as high. Thus, we can conclude that the LAR colony could provide an effective protection against predation of crows, magpies and gulls, dangers which accounted for the vast majority of STH nest failures in the year of our study.

We studied breeding success, chick growth, parental effort and chick behaviour in two groups of Lesser Black-backed Gulls Larus fuscus whose chicks were provided with additional food until 7 days after hatching or until fledging. These data were compared with those from control pairs which we studied simultaneously to test the hypotheses that food was in short supply during the chick stage at the colony site and that in such circumstances the behaviour of adults and young is mainly responsible for the low success. Pairs whose chicks were fed with additional food until fledging showed a higher fledging success than control pairs (intermediate for pairs of first experimental group). During the first week after hatching, experimental adults of both groups were present together at the territory for longer than control pairs. In adult females of experimental pairs, the length of feeding trips was shorter than in females of control pairs, whilst the rate of chick feeding was more frequent in the experimental broods. After the chicks were 7 days old, differences were significant only for the experimental pairs whose chicks were provided with additional food until fledging. Chicks fed until fledging showed a higher daily mass and wing-length increments and reached a higher fledging mass at an earlier age than both control chicks and chicks which were provided with additional food until day 7. Starvation occurred only in control chicks and in chicks of the first experimental group after we had stopped providing food. When food was in short supply, fledging success of gulls was adversely affected as a result of both starvation (because of the lower feeding rates of chicks) and a higher predation rate (arising from changes in behaviour of both adults and chicks).

In common gull colonies on islands of the Vistula River, Poland, adoption of chicks is common. In 1997, we observed 81 chicks from 35 nests. Of these, 19 (23.4%) left their natal broods and were adopted by other pairs. Another 11 (31.4%) were driven from the foreign territory by the owners. Foreign chicks were adopted by 15 pairs (42.9%). Eleven pairs (31.4%) drove foreign chicks from the territory. To test if the frequent adoptions in these colonies could be explained by kin selection or the occurrence of kin groups, we calculated band-sharing coefficients and genetic relatedness (r) between interacting birds (neighbours and non-neighbours). Adults that adopted were most often neighbours of the biological parents of adopted chicks, whereas spatially segregated birds, nesting further away, usually drove off the chicks. Band-sharing coefficients between males, but not females, were higher with decreasing internest distances. The band-sharing coefficients for adopted chicks and foster parents were significantly higher than for adopted chicks and randomly selected, spatially segregated pairs from the same and another colony. Band-sharing coefficients of adopted chicks and adopting neighbours (males: r=0.20; females: r=0.16) also tended to be higher than those of rejected chicks and rejecting neighbours (both sexes: r=0.08). Our results suggest that kin groups of neighbours do occur in common gull colonies. Such social structure might lead to indirect inclusive fitness benefits of adopting pairs. Differences in genetic similarity between chicks and adopting or rejecting neighbours show that at least in common gulls we should consider kin altruism as a factor in adoptions. Copyright 2000 The Association for the Study of Animal Behaviour.

During the last decade, the mating systems of many bird species have been analysed using molecular genetic methods. Most of these studies were performed on Passeriformes while Non-Passeriformes were rarely investigated. To fill this gap, we analysed blood samples of 24 Common Gull (Larus canus) families from two Polish colonies using DNA fingerprinting. A total of 55 of 56 chicks was unequivocally assigned to their genetic parents. Only two of these chicks (3.6%) from two different broods resulted from extra-pair copulations, the other 53 could be attributed to the putative parents; i.e., intraspecific brood parasitism was not observed. With respect to the number of breeding pairs investigated, the frequency of extra-pair paternity amounted to 8.3%. Thus, extra-pair paternity is relatively infrequent compared to other bird species. This result is surprising because the percentage of observed extra-pair copulations (i.e., mainly forced copulations in Common Gulls) in relation to the absolute number of copulations was 15.7% in the colonies studied. A possible reason for the disproportionate number of forced copulations and the lower percentage of resulting extra-pair young may include different mechanisms for paternity control by the pair males and/or by females.

Breeding success was studied in relation to time partitioning, and diet, in Herring Gulls (Larus argentatus) on Terschelling, The Netherlands, in 1992. The average fledging success of 0.5 chicks fledged/pair (n = 26 pairs) was lower than in the 1960s. We suspect that increased conspecific predation of eggs and chicks was the result of local food shortages. Successful pairs differed markedly in attendance patterns and diet from unsuc- cessful pairs. Unsuccessful pairs traveled to the feeding grounds more frequently (pre-egg and egg stages), and left the territory unguarded, for longer periods of time (egg and chick stages) than successful ones. Feeding frequency of chicks was significantly higher in successful pairs and was accompanied by higher growth rates in chicks. Overall, successful pairs ate more fish and chicks of conspecifics and Lesser Black-backed Gulls (L. fuscus) than unsuccessful ones, while the latter generally ate more crabs and starfish. Chicks of pairs that did not change their diet at hatching suffered higher mortality than chicks of pairs that did. We suggest that the deteriorating feeding conditions on Terschelling since the late 1960s has resulted in less synchronous territory attendance within pairs and less adequate feeding behavior, resulting in high levels of conspecific predation and relatively poor chick growth. Received 20 June 1995, accepted 20 August 1995.