Seasonal Decline In Reproductive Success Research Articles

Temporal changes in reproductive success and optimal breeding decisions in a long-distance migratory bird

Many avian migrants have not adjusted breeding phenology to climate warming resulting in negative consequences for their offspring. We studied seasonal changes in reproductive success of the greater snow goose (Anser caerulescens atlantica), a long-distance migrant. As the climate warms and plant phenology advances, the mismatch between the timing of gosling hatch and peak nutritive quality of plants will increase. We predicted that optimal laying date yielding highest reproductive success occurred earlier over time and that the seasonal decline in reproductive success increased. Over 25 years, reproductive success of early breeders increased by 42%, producing a steeper seasonal decline in reproductive success. The difference between the laying date producing highest reproductive success and the median laying date of the population increased, which suggests an increase in the selection pressure for that trait. Observed clutch size was lower than clutch size yielding the highest reproductive success for most laying dates. However, at the individual level, clutch size could still be optimal if the additional time required to acquire nutrients to lay extra eggs is compensated by a reduction in reproductive success due to a delayed laying date. Nonetheless, breeding phenology may not respond sufficiently to meet future environmental changes induced by warming temperatures.

Smart and safe? Antipredator behavior and breeding success are related to head size in a wild bird

Increased brain size has been associated with greater sensitivity to environmental context, but this flexibility is potentially costly as sampling the environment is time and energy consuming and may even increase the risk of predation. However, these potential trade-offs remain virtually unexplored in natural populations. We hypothesized that large brain size is 1) beneficial under challenging conditions and allows better matching of antipredator responses to the actual threat by predators and 2) associated with thorough risk assessment, which can be costly under benign conditions. To test these hypotheses, we examined the relationship between relative head volume, reproductive decisions, and fitness components in female common eiders (Somateria mollissima) under variable predation risk and breeding phenologies. This species is ideal for this purpose because of highly variable predation pressure and a distinct seasonal decline in reproductive success. The results were consistent with our hypotheses. First, females with depredated nests had smaller heads than expected by chance when predation rate (killed females/nest) was highest (challenging conditions). Second, large-headed females, but not small-headed ones, took a shorter time to form antipredator brood-rearing coalitions in more dangerous years. Third, large-headed females had a later onset of breeding, and their nests were more likely to be depredated when annual median nesting was earliest and predation pressure on females was low (benign conditions). Thus, predation risk and annual phenology may exert temporally fluctuating selection on relative head size, maintaining intraspecific variation in cognitive ability.

The timing of birds' breeding seasons: a review of experiments that manipulated timing of breeding

Reproductive success usually declines in the course of the season, which may be a direct effect of breeding time, an effect of quality (individuals with high phenotypic or environmental quality breeding early), or a combination of the two. Being able to distinguish between these possibilities is crucial when trying to understand individual variation in annual routines, for instance when to breed, moult and migrate. We review experiments with free-living birds performed to distinguish between the 'timing' and 'quality' hypothesis. 'Clean' manipulation of breeding time seems impossible, and we therefore discuss strong and weak points of different manipulation techniques. We find that the qualitative results were independent of manipulation technique (inducing replacement clutches versus cross-fostering early and late clutches). Given that the two techniques differ strongly in demands made on the birds, this suggests that potential experimental biases are limited. Overall, the evidence indicated that date and quality are both important, depending on fitness component and species, although evidence for the date hypothesis was found more frequently. We expected both effects to be prevalent, since only if date per se is important, does an incentive exist for high-quality birds to breed early. We discuss mechanisms mediating the seasonal decline in reproductive success, and distinguish between effects of absolute date and relative date, for instance timing relative to seasonal environmental fluctuations or conspecifics. The latter is important at least in some cases, suggesting that the optimal breeding time may be frequency dependent, but this has been little studied. A recurring pattern among cross-fostering studies was that delay experiments provided evidence for the quality hypothesis, while advance experiments provided evidence for the date hypothesis. This indicates that late pairs are constrained from producing a clutch earlier in the season, presumably by the fitness costs this would entail. This provides us with a paradox: evidence for the date hypothesis leads us to conclude that quality is important for the ability to breed early.

The quality and the timing hypotheses evaluated using data on great reed warblers

The seasonal decline in reproductive success observed in many animal species may be caused by timing per se (timing hypothesis) or by variation in phenotypic quality between early and late breeding females (quality hypothesis). To distinguish between these two hypotheses, several studies of birds have used clutch removal experiments to manipulate breeding date. However, removal experiments also increase the females’ previous reproductive effort due to the production of an extra clutch and a longer incubation period. According to life‐history theory an increase in reproductive effort lowers future reproduction. Hence, life‐history theory predicts lowered success of replacement broods for other reasons than expected from the timing hypothesis. Female great reed warblers, Acrocephalus arundinaceus, studied in Sweden are frequently exposed to nest predation, after which many lay replacement clutches. In order to examine possible effects of previous reproductive effort on different fitness components, we analysed the re‐laying frequency and the reproductive success of replacement broods in relation to time of the season and previous reproductive effort (measured as the length of the previous breeding attempt, LPB). In clutches of re‐laying females both the number of fledglings and the proportion of recruits were negatively correlated with LPB, whereas re‐laying frequency and clutch size were not related to LPB. We expect such relationships to be present also among other species. Consequently, the use of replacement clutches, as for example in clutch removal experiments, in evaluations of the cause of the often observed seasonal decline in various fitness components, might exaggerate the importance of the timing hypothesis over the quality hypothesis.

Seasonal variation in reproductive success and post-nuptial moult of blue tits in southern Europe: an experimental study.

Post-nuptial moult and reproductive success were studied in relation to timing of breeding in blue tits, Parus caeruleus, breeding in southern Europe. A group of experimentally delayed pairs was created by removing first clutches, thereby inducing late repeat clutches. Reproductive success and post-nuptial moult of delayed pairs were compared with both control pairs that bred early and unmanipulated late-breeding pairs. Delayed pairs fledged fewer young and with a lower body mass than control pairs. However, the number of fledged young and fledgling mass did not differ between delayed and late-breeding pairs. These results were more consistent with the date hypothesis, and it is concluded that the timing of breeding and reproductive success may be causally related in the blue tit. This study reveals a harmful effect of relaying on female body mass at the end of the nestling period. Therefore, females apparently pay the costs of relaying, since a reduction in body mass during the nestling period may be accompanied by a lowered survival probability. Delayed and late-breeding males often began moulting while still feeding young, but neither control males nor females from the three study groups did so. These results support the view that timing-related energy constraints on breeding may be important causes of a seasonal decline in reproductive success at different latitudes.

Fitness Components Associated with Laying Date in the Cliff Swallow

We report how clutch size, annual reproductive success, and annual survival of breeders and offspring vary with laying date in Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, from 1982-1993. Clutch size declined significantly across the season for both parasite-free (fumigated) and naturally infested nests. Larger clutches of 4 and 5 eggs were initiated significantly earlier in warm and dry seasons; laying dates for smaller clutches showed no relationship with weather. Incubation period declined significantly with laying date and was shorter in the absence of ectoparasites. Fledging success declined significantly with laying date in nonfumigated nests, primarily through greater loss of nestlings to parasitic swallow bugs (Oeciacus vicarius). First-year survival probabilities of offspring varied significantly with date among nonfumigated nests but not among fumigated nests; earlier-hatched young survived best in some years, and in other years intermediate- or late-hatched young had highest survival. Number of young locally recruited was highest for early nesters in 5 of 7 years with intermediate nesters doing best in the remaining years. Annual survival of female breeders increased with laying date, suggesting a benefit to late nesting and a potential life history trade-off between reproduction and adult survival. Nestling body mass did not vary with laying date, suggesting no decline in food availability over the summer. Female body mass prior to and during laying declined significantly with laying date but during brood rearing increased significantly with laying date. Female body mass had no significant effect on clutch size independent of laying date. The primary environmental factor causing the seasonal decline in reproductive success is ectoparasitism. A seasonal decline in clutch size may be a strategic adjustment to reduce ectoparasitism by lowering the parasite carrying capacity per nest. Greater apparent survival of late-nesting females may prevent directional selection for earlier breeding times in Cliff Swallows.

Intraspecific brood parasitism: a strategy for floating females in the European starling

In many bird species, there is a floating population of females that are excluded from breeding because of competition for limited breeding resources. Female floaters may enhance their reproductive success by engaging in intraspecific brood parasitism. We studied female floaters in a population of European starlings,Sturnus vulgaris, in order to determine their identity and potential parasitic behaviour. Females were caught after being attracted to nestboxes with artificial nests during 1993–1995. None of the females was known to have a nest of her own at capture but 47% of the females either laid an egg in the nest or carried a fully developed egg within the reproductive tract, indicating that they were intraspecific brood parasites. The floating females were significantly younger and smaller than breeding females. Of 13 females equipped with radiotransmitters and followed daily, all but one started a breeding attempt of their own after 3–8 days and the majority settled as secondary females or mated with males where the original female had disappeared. This suggests that females that are unable to compete successfully for nest sites or males early in the breeding season may use intraspecific brood parasitism to enhance reproductive success during the period that they are constrained from breeding. The importance of settling rapidly because of a seasonal decline in reproductive success may also promote the evolution of intraspecific brood parasitism in the starling. The relative reproductive success of combining egg dumping with breeding compared with traditional breeding will depend on the costs of delaying breeding as well as the probability of finding a mate later in the breeding season.

LIMITATION OF REPRODUCTIVE SUCCESS BY FOOD AVAILABILITY AND BREEDING TIME IN PIED FLYCATCHERS

Seasonal variation in reproductive success is a common phenomenon in many taxa. In birds, the costs and benefits associated with alternative breeding dates cause variation in clutch size, which is a main cause of seasonal decline in reproductive success. An immediate cause for this might be seasonal variation in the environment, or alternatively, but not mutually exclusively, date-independent differences in the parental/territory quality of early and late parents. The predictions of these hypotheses on seasonal reduction of reproductive success were tested by manipulating the hatching date and the amount of food during the nestling period of Pied Flycatchers (Ficedula hypoleuca). Delay in the hatching date caused lower fledging mass and lower fledging production, suggesting a causal relationship between breeding time and reproductive success. However, if delayed parents were provided with extra food, their success was the same as that of the controls. This suggests that deteriorating food supplies and/or higher energy demands for rearing the brood later in the season are important causes of the lower success of late breeders. Additionally, parental molt was affected by manipulation of both breeding date and food; delayed parents often began molting while still feeding young, but control parents generally did not. Furthermore, fledglings in the delayed group weighed less if their fathers began molting while feeding the young. This implies that parents traded off their onset of molt against the time and energy needed for their brood. These results support the view that timing-related energy constraints on breeding are important causes of seasonal decline in clutch size and reproductive success.

The Effect of Age on Timing of Breeding and Reproductive Success in the Thick‐Billed Murre

A seasonal decline in reproductive success is common to many birds breeding in seasonal environments. We examined the reproductive success of Thick—billed Murres breeding at Coats Island, Northwest Territories, Canada to assess the extent to which a seasonal decline in success could be accounted for by correlations between the age of the birds and their date of laying. In 1990 and 1991 reproductive success declined with date of laying: this decline was due mainly to a decline in the proportion of eggs that hatched. No corresponding decline was found in the proportion of hatchlings that reached fledging age. Pairs containing at least one young (4— or 5—yr—old) bird laid later and hatched a lower proportion of eggs than those containing at least one older (> 7—yr—old) bird. The two groups did not differ in the proportion of hatchlings reared. When looked at separately, the proportions of young and older birds that succeeded in hatching their egg were not affected by date of laying. When the two age groups were combined, the trends with date were similar to those found for a large sample, mostly birds of unknown age. To correct for differences in timing of laying, a sample of early breeders was induced to lay replacement eggs 14 d later. These experimental pairs were as successful as early control breeders. A greater proportion of the experimentally delayed pairs hatched chicks than birds laying for the first time at a similar date. The experimental pairs were also more successful than young birds that bred earlier. The growth of experimental and control chicks did not differ. However, the 14—d—old chicks of young pairs were lighter than those of experienced breeders, suggesting poorer provisioning by the young breeders. We concluded that there was no evidence for a deterioration in feeding conditions with date and that the relationship between age and/or experience and date of laying was the most important factor in causing the decline in reproductive success with date of laying was the most important factor in causing the decline in reproductive success with date. Our conclusions may apply to other studies of reproductive success in marine birds.

Seasonal Decline in Reproductive Success of the Great Tit: Variation in Time or Quality?

Seasonal variation in reproductive success is a common feature of most organisms. To understand the evolution of breeding seasons and reproductive strategies of individual animals, it is necessary to assess the extent to which seasonal variation in reproductive success is causally related to seasonal variation in the environment ('timing' hypothesis), to differences in quality between early and late breeders or their territories ('quality' hypothesis), or to a combination of both. We manipulated timing of breeding in the Great Tit Parus major, a small passerine, to test these hypotheses. A group of experimentally delayed birds was created by removing first clutches, inducing birds to lay a replacement clutch. Reproductive success of delayed pairs was compared with control pairs that bred early and with pairs that bred late. We conclude that seasonal declines in reproductive success at the nestling stage and survival of adult females were caused by differences in quality between early and late breeders. Recruitment of fledglings into the breeding population and the occurrence of second clutches were causally related to the timing of breeding. The seasonal decline in clutch size was caused by a combination of timing and quality effects. We attempted to assess the relative importance of timing and quality in the seasonal decline in reproductive success, expressed as lifetime production of recruits. We tentatively conclude that 87% of the seasonal decline in lifetime reproductive success could be attributed to a timing effect per se, whereas quality differences between early and late breeders accounted for the remaining 13%.

Seasonal Decline in Collared Flycatcher Ficedula albicollis Reproductive Success: An Experimental Approach

To assess the relationship between parental quality, seasonal environmental changes, and seasonal reproductive success, we manipulated both hatching time and clutch size among collared flycatchers (Ficedula albicollis) breeding on the island of Gotland, Sweden. Our experimental hatching delays of one week had no significant effect on the body condition of experimental females, relative to that of controls. In the absence of simultaneous clutch size reductions, experimental delays in hatching date caused significant decreases in fledging success relative to controls. However, when coupled with a 1-egg decrease in clutch size, experimental delays had no effect on fledging success. The results suggest that both parental/territory quality and environmental changes over the season affect the seasonal decline in reproductive success of birds

Experimental Evidence for a Causal Relationship between Timing and Success of Reproduction in the Great Tit Parus m. major

SUMMARY (1) We investigated experimentally whether timing of reproduction is causally involved in the seasonal decline in reproductive success in the great tit Parus m. major L. This is important because of its implications for the optimization of reproductive decisions of the individual bird. (2) Timing of breeding was manipulated by inducing repeat clutches through removal of the first clutch. The experiment was performed in 1987 in two forests (W and 0) and followed up the year after. It was assumed that parental quality was unaffected by the ex-periment. Reproductive success of the experimental and undisturbed control clutches were compared. For forest 0, reproductive success of the experimental clutches was also compared with the expected reproductive success of first clutches laid at the same time. (3) Clutch size, fledging success and nestling weight in experimental clutches were reduced as compared with controls in forest W but not in forest 0. We conclude that late breeding causes reduced pre-fledging reproductive success in forest W but not in forest 0. (4) Recruitment of fledged young in the breeding population was strongly reduced by the experiment. We conclude that late breeding caused reduced post-fledging reproductive success in both forests. (5) Survival of females was reduced by the experiment in forest 0 relative to controls and this corresponded with the natural seasonal trend. Survival of females in W and of males in both forests was unaffected. In experimental females, clutch size in the next breeding season was reduced by 16 eggs compared with controls. The clutch size of the males was unaffected, as was the following year's laying date for both sexes. We conclude that future reproductive success of females, but not of males, is also related to the timing of current reproduction. (6) Possible mechanisms underlying the relationship between date and reproductive success are discussed. It is suggested that differences in pre-fledging reproductive success induced by the experiment are related to food availability. Differences in post-fledging reproductive success are probably caused by competition between early and late young.