Temperature sensitivity of breeding phenology and reproductive output of the Common Redstart (Phoenicurus phoenicurus)
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Over the past four decades, rising temperatures have impacted the breeding phenology of many bird species, in some cases with consequences for their reproductive success. Migratory birds face particular challenges in shifting breeding phenology to track warmer springs, and understanding the impacts of rising spring temperatures on migratory birds' breeding is urgent. Here, we use over 4000 UK observations of Common Redstart nests, and spring temperature data from 1974 to 2020, to examine the effect of spring temperatures on laying date, clutch size and brood size. We use a sliding window approach to detect periods over which traits are most sensitive to temperature, and compare phenotypic responses to temperature over space and time with the aim of identifying causal effects of temperature and inferring the contributions of plasticity and local adaptation. We found that redstart laying date was sensitive to spring temperature from mid‐April to late May, with a relatively shallow response of 1–2 days/°C that was similar across space and time, but shallower than the phenological response of many of the resource species. Over the study period, laying date has advanced by more than 11 days, which is substantially more than can be explained based on the temperature plasticity estimates we obtained. Spring temperature had a weak, but positive, impact on clutch size, but with no evidence of an effect of spatial variation in temperature. The rate of brood size reduction from hatching to fledging became more negative at higher temperatures, but after taking into account a non‐significant but positive effect of temperature on brood size at hatching, there was no net effect of temperature on fledging success. Taken together, we found little evidence that higher temperatures in the UK lead to lower reproductive output.Keywords:
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Avian clutch size
Swallow‐tailed Gulls lay single‐egg clutches, and so raise single‐chick broods. As they are pelagic seabirds, this small brood size is expected to relate to proximate food limitation owing to infrequent food deliveries. However, a previous brood doubling experiment detected an 82% increase in fledging success from experimentally doubled broods compared to controls. We repeated the brood doubling experiment, and found that none of 50 enlarged broods produced more than one independent offspring. Control and experimental parents produced fledglings of similar body size, which also had indistinguishable rates of fledging and subsequent survival and reproduction. A variety of parameters estimating survival and breeding costs of reproduction showed no treatment effect. Since two‐chick broods yield dramatically higher fledging rates at some times, apparently without excess costs of reproduction, selection on brood size appears to favour a two‐chick brood. However, selection may not favour a two‐egg clutch if egg production is very costly. Additionally, our estimates of reproductive success do not incorporate the performance of experimental and control offspring as adults, which could differ, since growth of chicks differed slightly by treatment.
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Brood sizes in Pink-footed Geese and Greylag Geese were measured just after hatching, before fledging and after fledging in south and northeast Iceland in 1987 and post-fledging brood sizes were measured in south, north and northeast Iceland in 1988. In both years brood sizes were also measured after the birds' arrival in Scotland. Brood sizes after hatching were considerably lower than known clutch sizes in both species and Pinkfoot broods continued to decline until after fledging. There was no change in Pinkfoot brood size following migration but Greylag broods were smaller in Scotland than in Iceland in one of the two years of the study. Post-fledging brood sizes did not vary between areas in either species in 1988, or in Pinkfeet, between south and northeast Iceland in 1987. In Greylags, post-fledging brood size in 1987 was significantly higher in south Iceland than in the northeast, where egg harvesting occurred. It is suggested that the lack of any decline in breeding output as the populations of both goose species increased is likely to be due to extension of breeding range into new areas where breeding success was comparable to that in the former range.
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Anatidae
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The Swedish population of Wrynecks Jynx torquilla has decreased at least during the last decades. The reason can be worse breeding success, fewer breeding places, or problems in the wintering areas. This study compares brood size at ringing and number of ringed broods between the periods 1962—1981 and 1982—2001. Breeding success did not decrease. On the contrary, a small but significant increase was shown, from 6.7 to 7.1 young per brood. Despite this the number of ringed broods declined from 1,016 to 449. Deducting an observed 15% loss between ringing and fledging, 5.7–5.8 young would fledge from broods that were not deserted before ringing. Wrynecks lay about 10 eggs, so the loss to fledging is high, but despite this the observed brood size ought to be enough to maintain a stable population if mortality alone were to be compensated, indicating that habitat loss is the most likely explanation for the decline, probably diminishing area of pasture with fewer grazing cattle which in turn decrease the abundance of the ant species on which the Wrynecks feed.
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Summary The rates of clutch size, brood size and brood‐size fledged recorded during the period 1956≈63 (2 years interrupted) in the rural and the urban colony of the grey starling were analysed comparatively. The clutch size was significantly larger in the urban than in the rural one and the brood size was also larger significantly in the urban population. But, the difference in brood‐size fledged was insignificant. This reflected that the hatching rate was similar in both colonies (the rates in total differed but possible unusual rate was included in the clutch size of 5 eggs) and this may be determined physiologically but not depending upon food supply since smaller clutches showed higher hatching rate. However, the fledging rate was higher in the rural and lower and more variable in the urban colony. This is apparently due to food which was nutritive animal food in the rural but largely mixed with fruits in the urban. The fledging rate was rather irregularly variable with brood size suggesting that this is dependent upon parents' adaptability in feeding. However, the rate of 100% fledging becomes higher from brood sizes of 6 and less chicks. Thus, in general larger clutch and brood sizes produced larger absolute numbers of chicks and chicks fledged respectively. But, 5 was the most frequent size in the clutch and brood sizes and 4 in the brood‐size fledged. From the above, the ecological evolution of urban population was discussed.
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Brood size and hatching patterns in birds are widely believed to represent adaptations to food availability during the brood-rearing period. Experimental manipulations of brood size and hatching patterns have been widely employed to determine whether parents can raise larger or more synchronous broods, but concurrent manipulations of food abundance to test the supposed causal mechanism have been rare. I studied survival to fledging of 1,060 American Coot (Fulica americana) chicks from 99 broods for which brood size, hatching asynchrony, and food availability had been experimentally manipulated via inter-nest transfers of newly hatched young and provisioning of supplemental food. Survival of color-marked young was measured until 45 days posthatch using Cormack-Jolly-Seber mark—resighting models. Survival of offspring from unsupplemented broods declined linearly with experimental increases in brood size, and this decline was large enough to ameliorate any benefits to parents from larger broods. However, offspring survival was unaffected by experimental alterations of brood size in American Coots that received supplemental food, and supplemented pairs would have benefited from raising larger broods. Parents that produced larger clutches were more successful at raising large broods, consistent with the individual-optimization hypothesis. By contrast, observed hatching patterns were not optimal at promoting offspring survival, with both experimental increases and reductions in asynchrony leading to higher fledging rates. American Coot parents appeared to be adept at regulating food allocation among offspring with or without hatching asynchrony, which suggests that hatching patterns are most likely an artifact of selection for early onset of incubation.
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David Lack proposed that parental feeding ability ultimately limited clutch size in bird species in which the young were dependent upon their parents for food. However, many species can raise broods that are larger than their normal clutch size. Based on nine years of experimental results from an individually marked population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York (USA), I test six hypotheses that have been proposed as explanations for why birds fail to lay larger, seemingly more productive clutch sizes. I modified brood sizes by adding or removing 1–2 nestlings when broods were 1–3 d old and then documented the effects of brood size and manipulated brood size on nestling size and survivorship, offspring recruitment, adult survival, and future adult reproduction. Most first clutches of the season held three eggs (62% of 503 clutches), but the proportion of young to fledge did not vary with brood size (1–5 young), and as a result, broods of five were the most productive. Lack's basic food-limitation model was thus rejected. Although nestling mass and ninth-primary length at fledging declined with brood size, offspring survival during the immediate 10–12 d period after fledging was unrelated to nestling mass or lengths of the tarsus or ninth primary. The findings that the underweight young in broods of four and five did not suffer disproportionate mortality and that they were just as likely to appear as recruits in future years led to a rejection of the extended version of Lack's food-limitation model. Comparisons of annual variation in the relationship between productivity and brood size showed that productivity increased with brood size in eight of nine years (significant in six years). Thus, high temporal stochastic variation in conditions for rearing young (the “bad-years” hypothesis) is unlikely to explain the relatively small clutch size of kingbirds. Predictions of two other hypotheses that predict asymmetrically low survivorship of young in large broods (the “cliff-edge” and “brood-parasitism” hypotheses) were also rejected. On the other hand, evidence suggested that females individualize clutch size such that each female lays a clutch that matches her individual feeding ability. Although fledgling production was not adversely affected by experimental increases in brood size, most enlarged broods lost young during the 10–12 d immediately after fledging. Thus, enlarged broods ultimately produced no more independent young than did control broods that began with the same number of eggs. Fledgling deaths were not related to nestling mass or size, and recruitment was independent of manipulations. Survival and fecundity costs of reproduction also existed for females. Male survival (68%) was independent of the number of young that had been raised (0–5 young), and future breeding efforts were not compromised by elevated effort in the past year. However, females that raised broods of five were less likely to return to breed in the following year (42%) than were females that raised 2–4 young (62%). Among the survivors, females that raised enlarged broods in the preceding year also experienced more hatching failure and fledged fewer young than females that raised reduced broods in the preceding year. I suggest that costs of reproduction probably set the ultimate limit to clutch size in Eastern Kingbirds. I did not test the hypothesis that high rates of nest predation favor the evolution of small clutch size, but given that predators destroyed ∼50% of nests each year, it is also likely that nest predation has contributed to the evolution of the current clutch size of kingbirds. Whether a female produces a clutch of three or four eggs is probably determined by individual differences in parental ability, which may be related to either intrinsic properties of the female or territory quality.
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To study factors regulating clutch size in American Kestrels (Falco sparverius), brood manipulation experiments were performed on captive and wild birds in southwestern Quebec during 1986 and 1987. The largest normally occurring brood size was 5 young. Manipulations enlarged or decreased broods to 7 or 2 young, respectively. Significantly more young fledged from wild control and enlarged broods in 1987 than from comparable groups in 1986. The average number of young fledging from enlarged wild broods in 1987 was slightly higher than for control broods, but fledging weight was significantly depressed in enlarged broods. Growth rates and tarsal and antebrachial length at fledging were not affected by brood size, but development of primary feathers was slower in enlarged wild broods. Parental ability to adequately feed all young appears to be the major factor limiting brood size in American Kestrels.
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We assessed the post‐fledging survival of dippers Cinclus cinclus from 743 broods in relation to brood size, time of hatching and territory quality. We paid particular attention to assessing whether contrasting breeding performance along unproductive (i.e. acidic) and productive (i.e. circumneutral) rivers represented strategies which optimized the number of surviving young. For all brood sizes, post‐fledging survival varied significantly through the breeding season, with most survivors coming from attempts in the peak period of hatching. After correcting for these seasonal effects, the most common brood size overall, of four, was also the most productive as seen from post‐fledging survival; differences in the frequency of occurrence and survival between broods of four and five were marginal. Moreover, a change in the modal brood size from five to four occurred as the season progressed. consistent with a shift in brood productivity. Broods at acidic sites were significantly smaller than at circumneutral sites; while brood size four was the most productive at both types of site, brood size three was the second most productive at acidic sites, while brood size five was the second most productive at circumneutral sites. Dippers at acidic sites bred significantly later than at circumneutral sites, but post‐fledging survival declined most rapidly through the season at the former. These survival data provide evidence from both seasonal and spatial patterns that brood sizes in the dipper may be optimized in ways consistent with the enhancement of productivity. By contrast, delayed breeding at acidic sites contrasted with the patterns expected from optimization, instead reflecting resource scarcity.
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The possibility that parents of one sex may preferentially invest in offspring of a certain sex raises profound evolutionary questions about the relative worth of sons and daughters to their mothers and fathers. Post-fledging brood division—in which each parent feeds a different subset of offspring—has been well documented in birds. However, a lack of empirical evidence that this may be based on offspring sex, combined with the theoretical difficulty of explaining such an interaction, has led researchers to consider a gender bias in post-fledging brood division highly unlikely. Here we show that in the toc-toc, Foudia sechellarum, post-fledging brood division is extreme and determined by sex; where brood composition allows, male parents exclusively provision male fledglings, whereas female parents provision female fledglings. This is the first study to provide unambiguous evidence, based on molecular sexing, that sex-biased post-fledging brood division can occur in birds. Male and female parents provisioned at the same rate and neither offspring nor parent survival appeared to be affected by the sex of the parent or offspring, respectively. The current hypotheses predicting advantages for brood division and preferential care for one specific type of offspring are discussed in the light of our results.
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