Age Of First Breeding Research Articles

Population viability analysis predicts long‐term impacts of commercial Sooty Tern egg harvesting to a large breeding colony on a small oceanic island

The over‐exploitation of wild birds and the products derived from them can be a key threat driving changes in bird species richness and abundance. However, inadequate information on harvest levels combined with irregular population monitoring often means that the role of harvesting in population decline is difficult to quantify. Historically, the pan‐tropical Sooty Tern Onychoprion fuscatus has been subjected to extensive egg harvesting, yet the role of sustained harvesting in population change and future population viability remains unclear. In this study, we used published and new estimates of key demographic rates for a large, harvested Sooty Tern population in Seychelles, western Indian Ocean, to run a series of population viability analyses. We retrospectively assess the impact of historical levels of egg harvesting, and also predict how this population may respond under different future harvesting regimes, assuming no additional environmental change. We provide evidence that egg harvesting has played a substantial role in driving the population decline of Sooty Terns to date and demonstrate that continued harvesting will probably lead to further, possibly dramatic, declines in population size. These results indicate that recent levels of egg harvesting in Seychelles are not sustainable. We also show that the life‐history strategy of Sooty Terns, including a delayed age of first breeding, means the current 2‐year local moratorium on egg harvesting is unlikely to generate an observable population‐level response in Seychelles. Instead, we recommend that the current moratorium is extended at least beyond the age of first breeding (i.e. 5 years) to support appropriate evaluation. We additionally show that harvesting Sooty Tern eggs at much lower levels, i.e. 10% of the population size, is unlikely to reverse population decline. Therefore, long‐term egg harvesting strategies require careful evaluation to maintain a balance between the social, commercial, cultural and biodiversity significance of Sooty Terns in Seychelles.

Density‐dependent age of first reproduction as a key factor for population dynamics: stable breeding populations mask strong floater declines in a long‐lived raptor

AbstractThe age at which individuals reproduce for the first time is a key demographic factor impacting population dynamics and is subject to substantial variation across animal populations. There is also widespread empirical evidence that age of first reproduction responds to changes in population density over time, especially for long‐lived birds and mammals. The density dependence in age of first reproduction has profound impacts on the size of the non‐breeding (‘floater’) part of the population. A better understanding of floater dynamics in raptor populations is urgently needed for comprehensive assessments of conservation status and management. We use the European near‐endemic red kite Milvus milvus, a long‐lived raptor of conservation concern, as an example to investigate total population dynamics with an age‐structured demographic model. Using published estimates of survival and reproduction, we model the red kite population in Germany over four decades, considering also density dependence in the age of first reproduction in different model scenarios. Based on the literature and the results of our simulations, we show that age of first reproduction for the red kite most likely responds non‐linearly to density and that this general feedback mechanism should regularly be considered in demographic simulations. Our model results have far‐reaching implications for the conservation status of the red kite, as they highlight a drastic decrease of juvenile and non‐breeding individuals in the population over time – driven both by declining vital rates and a density‐dependent shift towards a younger age of first breeding. This process is not visible when judged only by the size of the breeding population, which our model estimates to be of similar size today as in the 1980s. The total red kite population reconstructed for Germany, however, seems diminished to nearly 50% of its former size.

Age of first breeding of Spoon-billed Sandpipers Calidris pygmaea

Age of first breeding of Spoon-billed Sandpipers Calidris pygmaea

Killing of raptors on grouse moors: evidence and effects

Owing to the intensity of game management in Britain, managers of grouse moors have illegally killed raptors to increase the numbers of Red GrouseLagopus l. scoticaavailable for shooting. This paper summarizes evidence for the recent scale of illegal raptor killing on grouse moors and its effects on populations. It provides insights into how raptors themselves respond demographically to different levels of killing. Over Britain as a whole, most raptors have increased and expanded considerably since the 1970s, in response to reduced killing and nest destruction, and the diminished impacts of organo‐chlorine pesticides; however, in recent decades the populations of some species have declined on and around grouse moors. This is widely evident in Hen HarrierCircus cyaneus, Peregrine FalconFalco peregrinusand Golden EagleAquila chrysaetospopulations and in more restricted areas also in Northern GoshawkAccipiter gentilisand Red KiteMilvus milvuspopulations, in all of which illegal killing has been sufficient to affect numbers over wider areas. The evidence consists mainly of: (1) greater disappearance of nesting pairs, lower breeding densities or reduced occupancy of apparently suitable traditional territories on grouse moors compared with other areas; (2) reduced nest success compared with other areas; (3) reduced adult survival compared with other areas; (4) reduced age of first breeding, reflecting the removal of adults from nesting territories and their replacement by birds in immature plumage; (5) greater levels of disappearance of satellite‐tracked birds on grouse moors than elsewhere; and (6) the finding of poisoned baits and traps, and shot or poisoned carcasses of raptors. Not all these types of evidence are available for every species, and other types of evidence are available for some. The Common BuzzardButeo buteois currently the most numerous raptor in Britain and also seems to be killed in the greatest numbers. Other raptor species, including MerlinFalco columbarius, Common KestrelFalco tinnunculusand Eurasian SparrowhawkAccipiter nisuswhich nest on or near grouse moors, have little or no significant impact on grouse and are killed less often or not at all. In the absence of illegal killing, some raptor species breed as well or better on grouse moors than in other habitats. Merlins, in particular, seem to thrive on grouse moors, benefiting from the management involved (including predator control). Other aspects of illegal raptor killing are discussed, including suggestions for ways in which it might be reduced.

Sounds of senescence: male swamp sparrows respond less aggressively to the songs of older individuals

Abstract Age-related changes in assessment signals occur in a diverse array of animals, including humans. Age-related decline in vocal quality in humans is known to affect perceived attractiveness by potential mates and voters, but whether such changes have functional implications for nonhuman animals is poorly understood. Most studies of age-related change in animal signals focus on increases in signal quality that occur soon after the age of first breeding (“delayed maturation”), but a few have shown that signal quality declines in older individuals after a mid-life peak (“behavioral senescence”). Whether other individuals are able to detect this senescent decline of assessment signals has not previously been tested. Here we use playback experiments to show that wild male swamp sparrows (Melospiza georgiana) respond more aggressively to songs from 2-year-old males as compared with songs from the same males when they are 10 years old. Senescence in signals that, like birdsong, affect reproductive success through intrasexual competition or mate choice may be of evolutionary significance.

Determining the optimal age of breeding gilts and its impact on lifetime performance, Literature review

Lifetime sow performance is a critical indicator for producers managing a commercial sow herd. It is comprised of sow longevity, lifetime piglets born alive, lifetime number of pigs weaned and lifetime non-productive sow days. Increased lifetime performance and longevity in sows reduces costs of replacement gilts and improves herd performance and profitability (Sasaki et al., 2008, 2011). One of the most critical factors driving the performance of sow herds is gilt management. Decisions regarding gilts have profound effects on sows’ lifetime performance. Age at first breeding of a gilt is a management decision that has been shown to affect performance and retention of the gilt in a herd.
 The main aim of this review is to establish factors affecting age of first breeding of gilts and its impact on lifetime performance.

Dispersal dynamics of juvenile Secretarybirds Sagittarius serpentarius in southern Africa

Knowledge of the dispersal behaviour of southern Africa’s juvenile Secretarybirds Sagittarius serpentarius was previously limited to a small number of ring recoveries (n = 4) in South Africa. From 2012 to 2015, 10 GSM-GPS solar trackers were fitted to juvenile Secretarybirds, aged approximately eight weeks, on their nests. The birds spent an average of 91.30 ± 8.80 d in their natal territories, which had a mean home range size (kernel density estimate 90%) of 1.21 ± 0.34 km2. As they aged, their exploratory movements around the nest expanded exponentially. During this period, exploratory activity peaked between 09:00 and 12:00. They showed high variability in maximum distance travelled from the nest and timing of dispersal from their natal territories. Most of the birds (n = 7) dispersed >150 km from their nests before returning to their natal region, suggesting the existence of natal philopatry in this species. The age of first breeding for a male Secretarybird was also confirmed during this study. The wide-ranging behaviour of juvenile Secretarybirds exposes them to a diverse range of threats, and a better understanding of their movements will aid the development of appropriate conservation strategies for this terrestrial raptor.

The role of age of first breeding in modeling raptor reintroductions.

The present biodiversity crisis has led to an increasing number of reintroduction programs, and this conservation method is likely to be increasingly used in the future, especially in the face of climate change. Many fundamental questions in population ecology are focused on the mechanisms through which populations escape extinction.Population viability analysis (PVA) is the most common procedure for analyzing extinction risk. In the use of PVA to model the trajectories of reintroduced populations, demographic values are sometimes taken from other existing wild populations or even from individuals in captivity.Density dependence in productivity is usually considered in viability models, but density‐dependent variation in age of first breeding is usually ignored. Nevertheless, age of first breeding has a buffering effect on population fluctuations and in consequence on population persistence.We simulated the viability of Spanish Imperial Eagle (Aquila adalberti) and Osprey (Pandion haliaetus) populations using data from established and reintroduced populations in southern Spain.Our results show that reduction in the age of first breeding is critical in the success of reintroductions of such long‐lived birds. Additionally, increases in productivity allow populations to growth at maximum rate. However, without considering variation in age of breeding, and the associated increasing overall productivity, reintroduced populations seem nonviable.To ignore density dependence in age of breeding in PVA means that we are seriously limiting the potential of the model population to respond to fluctuations in density, thereby reducing its resilience and viability. Variation in age of first breeding is an important factor that must be considered and included in any simulation model involving long‐lived birds with deferred maturity.

Hatchling survival to breeding age in Northern Pine Snakes (Pituophis melanoleucus) in the New Jersey Pine Barrens: Human effects on recruitment from 1986 to 2017.

To conserve threatened/endangered species, we need to understand the factors contributing to reproductive success and recruitment to reproductive stage. Obtaining this information is difficult for snakes because they are secretive, are not easy to locate at the same stage each year, and are sometimes sparsely distributed. We determined nest fate, hatchling growth and survival to age 5 years, and recruitment to breeding age of Northern Pine Snakes (Pituophis melanoleucus) in New Jersey Pine Barrens from 1986 to 2017. Pine Snakes are ‘threatened’ in New Jersey and in other states, and are at risk because of increased human population, habitat loss, predation, and poaching. Age of first-breeding was 4-years, based on snout-vent length of gravid and laying females, and snout-vent length of females followed as hatchlings to 5-years. Mean clutch size (+ 1 SE) was 9.5 + 0.3 (N = 53). The annual percent of nests in which eggs hatched averaged 25% (N = 288 nests), and varied among 5-year periods (5% to 30%/year). Of lab-reared hatchlings released into natal nests (N = 90), 26% (2015) and 32% (2016) reached hibernacula excavated in 2016 and 2017. The sex ratio of hatchlings reaching hibernation sites (N = 181) between 1986 and 2015 was skewed toward females (74/106, 59% females), and varied among 5-year periods (47–75% females). Once hatchlings reached a hibernaculum, there was a sex-related difference in survival. For hatchlings reaching a monitored hibernaculum, survival to 3-years was 35% in females and 40% in males, and to 4-years was 25% in females and 33% in males. Using these data, only 10% of females reached 3 years (first possible breeding age), and 7% survived to 4-years. Methodological problems with determining survival rates during these early critical years are discussed.

Age-related effects on breeding phenology and success of Common Guillemots Uria aalge at a North Sea colony

ABSTRACTCapsule: Common Guillemots Uria aalge show delayed breeding and marked age-related changes in reproductive success consistent with improved performance with experience.Aims: To determine age of first breeding and age-related effects on breeding phenology and success of Common Guillemots.Methods: Resighting data from a long-term colour-ringing study of Common Guillemot chicks were combined with observations of breeding phenology and success to follow the recruitment process, breeding phenology and success of 62 birds at a major North Sea colony over a 30-year period.Results: The median age of first breeding of Common Guillemots was 6.6 years. There were no detectable costs of first breeding on return rates or the likelihood of breeding the next season but first time breeders bred later and less successfully. Age of first breeding and lifetime breeding success both varied among individuals but there was no clear optimal age of first breeding and early first breeding was not associated with higher lifetime breeding success.Conclusions: Common Guillemots in the Isle of May population delayed breeding for 3–4 years beyond physiological maturity. The marked increase in breeding success with age was consistent with improved performance with experience rather than selection for higher quality individuals. Findings from this study will inform population models by providing improved estimates of age of first breeding and age-related changes in reproductive performance.

Survival of Sooty Falcons (Falco concolor) breeding in Oman

Although the Middle East supports a high level of avian biodiversity, the ecology of relatively few species that use the region has been studied in detail. Despite its restricted breeding distribution in the Middle East, and apparent unfavorable conservation status, little is known about the population ecology of the Sooty Falcon (Falco concolor), one of only two falcon species that breeds in the boreal summer. We applied multi-state models to capture–mark–recapture data collected during 2007–2014 in the Sultanate of Oman to estimate, for the first time, the probabilities of capture, age-specific breeding probabilities, and state-specific apparent survival for Sooty Falcon. Capture probability for breeding adults (±1SE) was 0.443 ± 0.088. Annual apparent survival probability for pre-breeders and for breeding adults was 0.570 ± 0.048 and 0.656 ± 0.069, respectively. The probability that 2-, 3-, and 4-year-old falcons returned as breeders was 0.065 ± 0.036, 0.159 ± 0.069, and 0.339 ± 0.211, respectively. In 2013, we radio-tagged five fledgling falcons, and monitored their fates using satellite-based tracking. All initiated their first migration and survived for 48 days following radio-tagging, but four of the five birds died by 70 days post-tagging; only one survived >100 days. Our results suggest that only about 12 % of fledglings survive to the average age of first breeding (~3.8 years), and that most of first-year mortality occurs during their first migration or soon after they reach their destination. Low apparent survival of pre-breeders could result in low recruitment to the breeding population, and population declines. A comprehensive population-level assessment is urgently needed to accurately determine the status of Sooty Falcons, and to devise flyway-scale conservation plans.

No effect of satellite tagging on survival, recruitment, longevity, productivity and social dominance of a raptor, and the provisioning and condition of its offspring

Summary The deployment of electronic devices on animals is rapidly expanding and producing leapfrog advances in ecological knowledge. Even though their effects on the ecology and behaviour of the marked subjects are potentially important, <10% of the studies are accompanied by an evaluation of impact, and comprehensive, long‐term assessments have been few. Therefore, there is an urgent need to test for impacts, especially for tags that are heavy and deployed for long time periods, such as satellite transmitters. We marked 110 individuals of a medium‐sized, migratory raptor, the black kite Milvus migrans, with GPS satellite tags, representing about 4% of the body mass and attached as backpacks through a Teflon harness. Tagged individuals were compared to control animals of similar sex, age and breeding status for a large number of behavioural, condition‐related and ecological traits. Despite a sample size two‐ to threefold greater than most previous assessments that reported significant impacts, there was no detectable difference between tagged and control individuals in key vital rates such as survival probability, longevity, recruitment, age of first breeding, reproductive performance and timing of breeding. Tagged and untagged kites showed similar social dominance during fights over food and a similar capability to provision nestlings, which prevented carry‐over effects on the stress levels and condition of their offspring. Synthesis and applications. Radio‐marking studies are growing exponentially in the current ‘movement ecology era’, and impact assessments will be ever more important. In principle, tags of up to 4% mass‐load can be deployed without apparent harm on some avian soaring species, but impacts should be properly evaluated on a case‐by‐case basis. Resilient species for which impacts seem weak could be used as early warning systems for trials of new devices: if impacts are observed, they are likely to be even greater on more vulnerable species. Finally, individual fatalities caused by marking should be taken into serious account, but comprehensively evaluated in the light of broader population‐level impacts. Future initiatives to minimize tagging impacts could include more stringent licensing criteria enforcing attendance at training courses or incorporation of impact evaluations into study designs, increased availability of training courses for tagging, and enhanced sharing of information through blogs, workshops or specialized journal sections.

British Ornithologists’ Union Union Medal

British Ornithologists’ Union Union Medal

Density dependence in seabirds: Great Skuas Stercorarius skua start to breed at a younger age when conditions are better

Large-scale and long-term ringing and cohort colour ringing of chicks, combined with trapping of immatures on clubs and adults on nests, allows the age of first breeding by Great Skuas Stercorarius skua to be investigated. Modal and median age of first breeding of Great Skuas at their largest colony (Foula, Shetland) was seven years, but there was considerable individual variation. Birds may first breed when four to >11 years old. Compared with the situation in Foula, Great Skuas start breeding at a younger age at smaller colonies. When conditions at Foula deteriorated, resulting in reduced adult survival, reduced breeding success and population decline, age of first breeding increased. These patterns imply a density-dependent age of first breeding, but with individual variation because higher-quality birds recruit at a younger age than birds in poor body condition. Density-dependent variation in age at first breeding may have an important influence on population growth rate, and this would merit study also in other species of seabirds through a dedicated ringing and colour-ringing programme.

The evolutionary significance of latent reproductive rate in a long-lived vertebrate.

Chambert, Rotella & Garrott () used long-term data to assess the evolutionary significance of individual latent reproductive rate in female Weddell seals. Latent reproductive rates capture the differences among individuals in terms of their propensity to breed; they are conceptual and mathematical constructs. Neither recruitment probability nor age of first breeding of daughters was related to the mother's latent reproductive rate, but there was evidence of a weak positive relationship between the latent reproductive rates of mothers and daughters, suggesting some degree of heritability in this trait. Females with a high latent reproductive rate were expected to produce 2·0 times as many recruited females and 2·1 times as many grandchildren as females with a low reproductive rate. There was substantial stochastic variation in the number of offspring and grandchildren produced, but the inter-individual variability in female latent reproductive rate may have important fitness consequences.

Age of First Breeding Interacts with Pre- and Post-Recruitment Experience in Shaping Breeding Phenology in a Long-Lived Gull

Individual variation in timing of breeding is a key factor affecting adaptation to environmental change, yet our basic understanding of the causes of such individual variation is incomplete. This study tests several hypotheses for age-related variation in the breeding timing of Lesser Black-backed Gulls, based on a 13 year longitudinal data set that allows to decouple effects of age, previous prospecting behavior, and years of breeding experience on arrival timing at the colony. At the population level, age of first breeding was significantly associated with timing of arrival and survival, i.e. individuals tended to arrive later if they postponed their recruitment, and individuals recruiting at the age of 4 years survived best. However, up to 81% of the temporal variation in arrival dates was explained by within-individual effects. When excluding the pre-recruitment period, the effect of increasing age on advanced arrival was estimated at 11 days, with prior breeding experience accounting for a 7 days advance and postponed breeding for a 4 days delay. Overall, results of this study show that delayed age of first breeding can serve to advance arrival date (days after December 1st) in successive breeding seasons throughout an individual’s lifetime, in large part due to the benefits of learning or experience gained during prospecting. However, prospecting and the associated delay in breeding also bear a survival cost, possibly because prospectors have been forced to delay through competition with breeders. More generally, results of this study set the stage for exploring integrated temporal shifts in phenology, resource allocation and reproductive strategies during individual lifecycles of long-lived migratory species.

Philopatry, natal dispersal, first settlement and age of first breeding of bearded vultures Gypaetus barbatus in central Pyrenees

Capsule Male bearded vultures tended to disperse closer to natal territories than females with a preferred eastward natal dispersal direction. In general, birds settled for the first time in breeding territories at an average age of 7.6 years, first breeding attempt occurred at 10.0 years and first successful breeding at 10.4 years. Measures aimed at favouring the settlement of new pairs in the edge of their current distribution area would allow natural range expansion of this endangered species.

The impact of reproductive investment and early‐life environmental conditions on senescence: support for the disposable soma hypothesis

Several hypotheses have been put forward to explain the evolution of senescence. One of the leading hypotheses, the disposable soma hypothesis, predicts a trade-off, whereby early-life investment in reproduction leads to late-life declines in survival (survival senescence). Testing this hypothesis in natural populations is challenging, but important for understanding the evolution of senescence. We used the long-term data set from a contained, predator-free population of individually marked Seychelles warblers (Acrocephalus sechellensis) to investigate how age-related declines in survival are affected by early-life investment in reproduction and early-life environmental conditions. The disposable soma hypothesis predicts that higher investment in reproduction, or experiencing harsh conditions during early life, will lead to an earlier onset, and an increased rate, of senescence. We found that both sexes showed similar age-related declines in late-life survival consistent with senescence. Individuals that started breeding at a later age showed a delay in survival senescence, but this later onset of breeding did not result in a less rapid decline in late-life survival. Although survival senescence was not directly related to early-life environmental conditions, age of first breeding increased with natal food availability. Therefore, early-life food availability may affect senescence by influencing age of first breeding. The disposable soma hypothesis of senescence is supported by delayed senescence in individuals that started breeding at a later age and therefore invested less in reproduction.

Ecological Specialization to Fluctuating Resources Prevents Long-Distance Migratory Raptors from Becoming Sedentary on Islands

BackgroundThe adaptive transition between behavioral strategies, such as the shift from migratoriness to sedentariness, remains an outstanding question in evolutionary ecology. Density-dependent variation in the age of first breeding has been proposed as a feasible mechanism through which long-lived migratory birds with deferred sexual maturity should become sedentary to persist on islands. Although this pattern seems to hold for most raptors and herons, a few exceptions have been identified. One of these exceptions is the Eleonora’s falcon, a long-distance migratory bird, which shows one of the most peculiar adaptations in the timing of reproduction and food requirements among raptors.Methodology/Principal FindingsHere, we compiled data concerning demography, banding recoveries and satellite tracking of Eleonora’s falcons to discuss likely explanations for the exceptional behavior of this insular long-distance migratory species.Conclusions/SignificanceNew data reveal that Eleonora’s falcons do return to the natal colonies in their first year and young birds are able to breed. However, in contrast to previous hypothesis, the highly specialized strategy of this and other ecologically similar species, as well as the virtual lack of food during winter at breeding areas prevent them from becoming sedentary on islands. Although the ultimate mechanisms underlying the process of sedentarization remain poorly understood, the evidence provided reveal the existence of important trade-offs associated with ecological specialization that may become particularly relevant in the present context of global change.

Experience versus effort: what explains dynamic heterogeneity with respect to age?

Age‐related patterns of survival and reproduction have been explained by accumulated experience (‘experience hypothesis’), increased effort (‘effort hypothesis’), and intrinsic differences in phenotypes (‘selection hypothesis’). We examined the experience and effort hypotheses using a 40‐year data set in a population of Leach's storm‐petrels Oceanodroma leucorhoa, long‐lived seabirds for which the effect of phenotypic variation has been previously demonstrated. Age was quantified by time since recruitment (‘breeding age’). The best model of adult survival included a positive effect of breeding age (1, 2, 3+ years), sex (male > female), and year. Among‐individuals variation (fixed heterogeneity) accounted for 31.6% of the variance in annual reproductive success. We further examined within‐individual patterns in reproductive success (dynamic heterogeneity) in the subset of individuals with at least five breeding attempts. Three distinct phases characterized reproductive success – early increase, long asymptotic peak, late decline. No effect of early reproductive output on longevity was found, however, early success was positively correlated with lifetime reproductive success. Reproductive success was lower earlier than later in life. Among the few natally philopatric individuals in the population, age of first breeding had no effect on longevity, lifetime reproductive success, or early reproductive success. No support for the effort hypothesis was found in this population. Instead, age‐specific patterns of survival and reproduction in these birds are best explained by the experience hypothesis over and above the effect of intrinsic differences among individuals.