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Climate change affects timing of reproduction in many bird species, but few studies have investigated its influence on annual reproductive output. Here, we assess changes in the annual production of young by female breeders in 201 populations of 104 bird species (N = 745,962 clutches) covering all continents between 1970 and 2019. Overall, average offspring production has declined in recent decades, but considerable differences were found among species and populations. A total of 56.7% of populations showed a declining trend in offspring production (significant in 17.4%), whereas 43.3% exhibited an increase (significant in 10.4%). The results show that climatic changes affect offspring production through compounded effects on ecological and life history traits of species. Migratory and larger-bodied species experienced reduced offspring production with increasing temperatures during the chick-rearing period, whereas smaller-bodied, sedentary species tended to produce more offspring. Likewise, multi-brooded species showed increased breeding success with increasing temperatures, whereas rising temperatures were unrelated to reproductive success in single-brooded species. Our study suggests that rapid declines in size of bird populations reported by many studies from different parts of the world are driven only to a small degree by changes in the production of young., This meta-analysis was financed by the grant of the Polish National Science Centre (Narodowe Centrum Nauki) (no. 2017/27/B/NZ8/00465) awarded to Lucyna Hałupka.

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.

Simple Summary Decades of selective breeding carried out on fur farms have changed the morphology, behavior and other features of the American mink, thereby differentiating farm and feral animals. The uniqueness of this situation is not only that we can observe how selective breeding phenotypically and genetically changes successive generations, but also that it enables a comparison of farm minks with their feral counterparts. Such a comparison may thus provide valuable information regarding differences in natural selection and selective breeding. In our study, we found significant morphological differences between farm and feral minks as well as changes in body shape: trapezoidal in feral minks and rectangular in farm minks. Such a clear differentiation between the two populations over a period of several decades highlights the intensity of selective breeding in shaping the morphology of these animals and gives an indication of the speed of phenotypic changes and the species’ plasticity. This also suggests that the selective forces (selective breeding vs. natural selection) acting upon body dimensions of minks vary between feral and farm populations. Abstract In this study, we performed a comparative analysis of the morphological traits between feral (n = 43) and farm (n = 200) individuals of the American mink in Poland to address the question of how multigenerational intensive selective breeding has morphologically differentiated these two populations. Nine body measurements and two proportion coefficients were obtained using adult individuals. The significance of differences between population means was assessed using the Wilcoxon test for independent samples, while the Kruskal–Wallis test was used to compare sex-population groups. Spearman’s correlation coefficients between measurements were estimated for each population. We also performed Principal Component Analysis (PCA) to identify the variables that were most closely correlated with variation in the trait measurements and to investigate the morphological differences between farm and feral minks. We found that the farm minks exhibited significantly higher mean values for eight out of eleven studied traits. Moreover, significant changes in forelimb length, with no concomitant changes in hindlimb length, were accompanied by differences in body shape: trapezoidal in feral minks and rectangular in farm minks. The PCA suggested an almost complete separation of the two populations and indicated that sexes were quite separate; farm males in particular constitute a wholly discrete cluster. Such a clear differentiation between the two populations and sexes over a period of several decades highlights the intensity of selective breeding in shaping the morphology of these animals.

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.

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.

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.

The study was conducted in the years 1994-2006 in the middle of the Vistula River between Wróble and Kochów (416th – 418th km of the waterway). The presence of islands and steep banks makes it a key place for nesting avifauna, including a number of endangered species. The aim of this paper is to present the impact of hydro-technical facilities (a diversion weir) on habitat changes, and consequently on the abundance and distribution of birds that inhabit this part of the river. A cross-divider in the riverbed (weir) connecting the bank of the river with one of the islands increased the flow rate and pushed the current to the middle of the river, directing it towards the islands. The result of these changes was a sudden erosion of the sandy, the high slope at the top of the island II, a partition of this island into two parts, and a slow erosion of the island located near the left bank (island IV, Fig. 1B). Lack of fixed, steady flow along the right bank resulted also in a considerably faster succession of vegetation on the island I and slow shallowing of the channel between the island and the bank. As a result, the area of sandy habitats, most valuable to nesting birds, decreased disproportionately faster than the area of the whole islands (Table 1). In the years 2001-2002, when the island I was completely united with the right bank and the islands 0 and I were connected to each other, a width of the riverbed at this point decreased from 870 m to 710 m, i.e. by more than 20 % (Fig. 1).