Post-fledging Mortality Research Articles

Post-fledging behavioral ecology of migratory songbirds: how do fledgling activity rates vary across species?

AbstractDespite a large literature on the ontogeny of animal behavior, our understanding of behavioral development in young songbirds remains relatively poor. This is particularly true for the post-fledging period, where it is often difficult to follow and observe small, freely moving fledglings for extended periods of time. Fortunately, technological advances have enabled more robust examinations of songbird post-fledging behaviors. To provide greater insight into fledgling developmental and behavioral ecology, we used automated radio-telemetry systems to document post-fledging activity rates of juveniles across 9 songbird species of east-central Illinois, USA. Specifically, we sought to examine differences in the ontogeny of activity rates across species, and compare activity rates to intrinsic (e.g. morphological traits, age) and extrinsic (e.g. predation risk, brood size) factors that have the potential to influence fledgling behavior. While post-fledging activity rates differed by species, our findings show remarkable similarities in the ontogeny of fledgling activity across the avian community; all species exhibited a steady increase in activity rates with post-fledging age, with rates leveling off roughly 3 to 4 weeks post-fledging. These general patterns mirrored rates of post-fledging mortality (primarily due to predation) across the period, including the presence or absence of a post-fledging bottleneck, in which species with bottlenecks exhibit slight declines in activity rates during the first few days out of the nest. Alongside fledgling age and mortality risk, our findings also suggest that juvenile wing development and brood size at fledging mediate fledgling activity rates within and across species.

Early-life patterns of growth are linked to levels of phenotypic trait covariance and postfledging mortality across avian species.

Life history studies have established that trade‐offs between growth and survival are common both within and among species. Identifying the factor(s) that mediate this trade‐off has proven difficult, however, especially at the among‐species level. In this study, we examined a series of potentially interrelated traits in a community of temperate‐zone passerine birds to help understand the putative causes and consequences of variation in early‐life growth among species. First, we examined whether nest predation risk (a proven driver of interspecific variation in growth and development rates) was correlated with species‐level patterns of incubation duration and nestling period length. We then assessed whether proxies for growth rate covaried with mean trait covariance strength (i.e., phenotypic correlations ( rp), which can be a marker of early‐life stress) among body mass, tarsus length, and wing length at fledging. Finally, we examined whether trait covariance strength at fledging was related to postfledging survival. We found that higher nest predation risk was correlated with faster skeletal growth and that our proxies for growth corresponded with increased trait covariance strength ( rp), which subsequently, correlated with higher mortality in the next life stage (postfledging period). These results provide an indication that extrinsic pressures (nest predation) impact rates of growth, and that there are costs of rapid growth across species, expressed as higher mean rp and elevated postfledging mortality. The link between higher levels of trait covariance at fledging and increased mortality is unclear, but increased trait covariance strength may reflect reduced phenotypic flexibility (i.e., phenotypic canalization), which may limit an organism's capacity for coping with environmental or ecological variability.

Pre- to post-fledging carryover effects and the adaptive significance of variation in wing development for juvenile songbirds.

Evolutionary ecologists have long been interested in the adaptive significance of morphological traits across stages of animal life. In some cases, traits that are not adaptive in one life stage may be adaptive in a subsequent stage. As such, morphological traits may generate important carryover effects, whereby conditions experienced during one life-history stage influence fitness during subsequent stages. Carryover effects are particularly relevant in young animals, as early life stages are thought to be critical with respect to animal life-history evolution and population dynamics. In songbirds, pre- to post-fledging carryover effects operating within species may be critical for survival and shape life histories among species, but remain poorly understood. Among potential songbird traits, wing development and its associated flight ability may be the most important for post-fledging survival. Thus, to assess the adaptive significance of wing development for juvenile songbirds under Arnold's (Integrative and Comparative Biology, 23, 1983, 347) classic performance-morphology-fitness paradigm, we tested for pre- to post-fledging carryover effects among 20 coexisting species (nine focal species) of an avian community in east-central Illinois, USA. We found evidence for pre- to post-fledging carryover effects of wing development in all species, by which individuals with less developed wings exhibited poorer flight ability and experienced higher rates of mortality after fledging. Furthermore, our findings suggest that carryover effects operating at the species level ultimately help shape patterns of life-history variation among species. Specifically, we found that species with higher rates of nest predation had shorter nestling periods, fledged young with less developed wings and exhibited higher rates of post-fledging mortality. Our results highlight the adaptive significance of wing development as a key factor generating pre- to post-fledging carryover effects among songbirds, and demonstrate how morphological traits, locomotor performance, and age-specific survival may trade-off and interact across juvenile life stages to shape animal life histories.

Post‐fledging survival of altricial birds: ecological determinants and adaptation

For altricial young, fledging is an abrupt step into an unknown environment. Despite increasing numbers of studies addressing the post-fledging period, our current knowledge of the causes and consequences of post-fledging survival remains fragmentary. Here, we review the literature on post-fledging survival of juvenile altricial birds, addressing the following main questions: Is low post-fledging survival a bottleneck in the altricial reproductive cycle? What is known of proximate and ultimate causal factors such as trophic relations (food and predation), habitat conditions, or abiotic factors acting in the post-fledging period? We analyzed weekly survival estimates from 123 data series based on studies of 65 species, covering weeks 1–13 post-fledging. As a general pattern, survival of fledglings was low during the first week post-fledging (median rate = 0.83), and improved rapidly with time post-fledging (week 4 median rate = 0.96). For ground-nesting species, survival immediately after leaving nests was similar to egg-to-fledging survival. For species breeding above-ground, survival during the first week post-fledging was substantially lower than during both the nestling period and later post-fledging stages. Thus, the early post-fledging period is a bottleneck of markedly elevated mortality for most altricial species. Predation was the main proximate cause of mortality. Various factors such as habitat, annual and seasonal variation in the environment, and the physical condition of fledglings have been found to affect post-fledging survival. Individual survival depended strongly on physical traits such as mass and wing length, which likely influence the ability of fledglings to escape predation. Trophic relationships at various levels are the main ultimate driver of adaptation of traits relevant to survival during the pre- and post-fledging periods. Spatiotemporal dynamics of food resources determine the physical development of juveniles and, in turn, their performance after fledging. However, predators can cause quick and efficient selection for fledgling traits and adult breeding decisions. Parental strategies related to clutch size and timing of breeding, and the age and developmental stage at which young fledge have substantial effects on post-fledging survival. The intensity and duration of post-fledging parental investment also influences fledgling survival. Post-fledging mortality is therefore not a random and inevitable loss. Traits and strategies related to fledging and the post-fledging stage create large fitness differentials and, therefore, are integral, yet poorly understood, parts of the altricial reproductive strategy.

Food supplementation does not increase demographic rates in a passerine species of conservation concern

Numerous studies have examined the effects of the provision of supplementary food on aspects of avian reproductive success, but far fewer have gone on to examine the potential positive effects of food supplementation on the demographic rates which are key for population growth rate. Testing for potential effects of food shortage on vital rates is likely to be particularly important in species of high conservation concern, where populations are particularly small, isolated or decreasing rapidly. Here we test the effects of the provision of supplementary food on reproductive success, body condition at fledging and post-fledging survival of ring ouzels (Turdus torquatus), a species of high conservation concern in the UK. However, food supplementation had no detectable effect on any of these parameters. There was no significant difference in return rates of fed and unfed fledglings in the year following hatching, and most post-fledging mortality was apparently caused by predation by raptors and mustelids. We conclude that the supply of invertebrate food sources for nestlings was not a major limiting factor in our study area, at least during this two-year study. Further studies are required to quantify the precise mix of habitats used by ring ouzels, at the appropriate scale, which provide concealment from predators and access to food supplies throughout the spring and summer months.

Importance of Juvenile Mortality in Birds' Population: Early Post-Fledging Mortality and Causes of Death in White StorkCiconia ciconia

ABSTRACT: Early post-fledging mortality of White Stork during the period from the first flight to their autumn departure from natal area was described. Data from eight breeding seasons (2005–2012) about 56 cases of death of juvenile White Storks from a study area in Western Poland were collected. For a comparison the data of 55 cases of death from recoveries data deposited in Polish Bird Ringing Centre from entire Poland were included in analysis. Early post-fledgling mortality rate varied from 2 to 11% (mean 4.3%) of all fledglings per year. The main causes of death were collision with power lines and electrocution which in Western Poland constituted 60% and in entire Poland — 78% of death cases with known causes. Other anthropogenic causes constituted responsibly 25 and 13% and natural causes — 15 and 9%. Mean distance where dead storks were found was 183.3m from natal nests, while 73% were found closer than 100 m from their nests.

A Molecular Genetic Assessment of Sex Ratios from Pre-fledged Juvenile and Migrating Adult Common Loons (Gavia immer)

Abstract. We used a polymerase chain reaction (PCR)-based sex identification technique to evaluate the sex ratios of pre-fledged juvenile Common Loons (Gavia immer) from three breeding populations in northern Michigan and northern Wisconsin, and of dead adult Common Loons migrating through Lake Huron. Results of these analyses show that more juvenile males than females fledged from all three breeding populations, although none of these biases differed significantly from parity. The sex ratio of migratory adults was at parity. No parental or territory quality indicators were significantly associated with the sex of chicks fledged from one intensely studied northern Michigan population at Seney National Wildlife Refuge. A significant male bias in adult re-observation of Common Loons banded as juveniles at the wildlife refuge was not solely attributable to the modest male bias recorded at fledging, and thus suggests a pattern of female-biased post-fledging mortality and/or female-biased adult dispersal in Co...

Translocation and hand-rearing of the short-tailed albatross Phoebastria albatrus: early indicators of success for species conservation and island restoration

AbstractMany endemic species, particularly those on remote islands, have been driven to extinction or near extinction by anthropogenic influences. The short-tailed albatross Phoebastria albatrus once numbered in the millions but was thought to be extinct by the mid 20th century. Albatrosses, of the family Diomedeidae, are among the most threatened birds globally as a result of commercial exploitation, introduced predators, and mortality in commercial fisheries. We applied an experimental approach over 5 years to evaluate the translocation and hand-rearing of albatross chicks by comparing growth, physiological health indices, post-fledging survival, and migration patterns with a control group of naturally reared chicks in the source population. Hand-reared chicks had comparable or superior health and similar rates of immediate post-fledging mortality (15%), with mortality strongly female-biased in both groups. Hand-reared birds had longer post-fledging drift periods before attaining sustained flight (also female-biased) but comparable, albeit somewhat wider ranging, migration patterns to naturally reared chicks during their first 6 months at sea. Recruitment to the translocation site of a breeding pair that included a hand-reared bird occurred within 5 years of the first translocation. Success will ultimately depend on continued recruitment and breeding over the coming decades, given delayed breeding in these long-lived species. The results to date, however, have exceeded initial expectations and can inform potential reintroductions of other long-lived, migratory avian species with strong natal philopatry, and reintroductions of native species to former breeding islands.

Postfledging survival, movements, and dispersal of Ring Ouzels (Turdus torquatus)

ABSTRACT. Recent evidence suggests that avian population growth rates may be constrained by low postfledging survival. Therefore, quantifying postfledging mortality and understanding the ecological factors that influence it are fundamental for assessing the relative importance of this life-history stage for population growth and identifying the ecological drivers of population dynamics. We radiotracked 110 juvenile Ring Ouzels (Turdus torquatus), a species of high conservation concern in the United Kingdom, to test hypotheses regarding the timing and causes of postfledging mortality and to quantify the timing and magnitude of local movements and dispersal. Juveniles fledged from early-season broods had higher survival during each 4-day period over 116 days postfledging (0.952 ± 0.011 [SE]) than juveniles fledged from late-season broods (0.837 ± 0.021). Most mortality occurred within the first 3 weeks postfledging, and predation by raptors and mammals was the main apparent cause of mortality, accounting fo...

Reproduction and Population Dynamics of HawfinchesCoccothraustes coccothraustesin the Primeval Forest of Białowieża National Park (NE Poland)

The breeding performance of Hawfinches was studied during seven years (1991–98, except 1995) under conditions of an extensive close-to-pristine oak-lime-hornbeam forest in the Białowieża National Park, eastern Poland. Two 30–31.5 ha plots were regularly checked each year to find most nests present, usually observed since the period of their construction. Mean clutch size (5.27 ± 0.66, for best year — 5.5) finally produced small family size, owing to a partial loss, fledging and post-fledging mortality. Average breeding losses calculated traditional way were 72.8% (n = 202), mostly due to egg robbing, then predation on nestlings (three times less frequent), and, sporadically, adverse weather conditions at the moment of fledging. Nesting success (5.9–35.7%, 27.2% on average), strongly varying between years, is lower than in most Hawfinch populations from other (anthropogenic) habitats, being one of the lowest among temperate Passerines. In spite of low production of young the species remains numerous across deciduous stands of the Białowieża Forest, with its numbers even increasing since the 1980s. This large and dense population living in an apparently optimal habitat may, sporadically, be supported by influxes from other (anthropogenic?) sites.

Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.

Post-fledging survival and dispersal of shy albatross from three breeding colonies in Tasmania

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 405:271-285 (2010) - DOI: https://doi.org/10.3354/meps08590 Post-fledging survival and dispersal of shy albatross from three breeding colonies in Tasmania R. Alderman1,2,*, R. Gales1, A. J. Hobday2,3, S. G. Candy4 1Department of Primary Industries, Parks, Water and Environment (DPIPWE), Hobart, Tasmania 7001, Australia 2Quantitative Marine Science Program and School of Zoology, University of Tasmania, Hobart, Tasmania 7001, Australia 3Wealth from Oceans Flagship, CSIRO Marine and Atmospheric Research, Hobart, Tasmania 7001, Australia 4Australian Antarctic Division, Kingston, Tasmania 7050, Australia *Email: rachael.alderman@dpipwe.tas.gov.au ABSTRACT: Limited knowledge of the movements of post-fledging albatross represents a significant gap in understanding albatross biology and conservation. Without clearer understanding of at-sea distribution and mortality during this life-history stage, the threats to albatrosses cannot be managed appropriately. We investigated this early at-sea behaviour of shy albatrosses Thalassarche cauta, which breed only in Tasmania. We deployed 48 satellite transmitters on fledgling birds from each of the 3 Tasmanian populations over 4 separate seasons. We observed population differences in the at-sea distribution, with the northern Albatross Island population foraging exclusively in southern Australian shelf waters to the west of the breeding colony. Birds from the 2 populations in southern Tasmania (Mewstone and Pedra Branca) also favoured these areas; however, they showed greater tendency to traverse the high seas and forage further west. These differences in spatial distribution mean populations have different exposure to fisheries and consequent risk of bycatch. Analysis of the satellite data and supporting evidence from band recoveries showed that juvenile mortality is highest in the period immediately after fledging. We speculate that this is related to foraging failure of naive birds. Differences between the 3 populations in post-fledging mortality were apparent. Albatross Island birds have greater chance of surviving the crucial initial learning period after fledging than either of the 2 southern populations, possibly due to proximity to food resources. KEY WORDS: Satellite tracking · Foraging grounds · Juvenile albatross · Survival · Thalassarche cauta Full text in pdf format PreviousNextCite this article as: Alderman R, Gales R, Hobday AJ, Candy SG (2010) Post-fledging survival and dispersal of shy albatross from three breeding colonies in Tasmania. Mar Ecol Prog Ser 405:271-285. https://doi.org/10.3354/meps08590 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 405. Online publication date: April 29, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Long-Term Study on Interactions between Tawny OwlsStrix aluco, JackdawsCorvus monedulaand Northern GoshawksAccipiter gentilis

We monitored the dynamics of owls, raptors and corvids within the 3400 ha of the Amsterdam Water Supply Dunes in the western Netherlands from 1961 through 2007. In the 1930s and 1950s, afforestation projects were initiated. In 1961 the first Tawny Owl nest was found, from whence the population increased steadily to stabilize in the 1970s at c. 24 pairs. Tawny Owl dynamics were affected by competition for nest sites with Jackdaws Corvus monedula and depredation by Northern Goshawks Accipiter gentilis. Jackdaws were able to take over nest boxes from egg laying and incubating Tawny Owls, reducing the letter's breeding success. Tawny Owl numbers and breeding success were negatively correlated with Jackdaw numbers (R2 = 0.556 and R2 = 0.592, respectively). After the first nest in 1993, Goshawk numbers rapidly increased to 10–11 pairs in 1999. The number of Jackdaw breeding pairs correlated negatively with the number of Goshawks (R2 = 0.661). Goshawks apparently limited Jackdaw numbers, which led to an improvement of Tawny Owl breeding success. However, post-fledging mortality of Tawny Owl from Goshawk predation was also significant, reducing local recruitment in the Tawny Owl breeding population.

Chick Survival, Fledgling Residency and Evaluation of Methods for Estimating Fledging Success in Least Terns

Fledgling counts are often used to estimate fledgling production or calculate an index of chick survival (i.e., fledging success) in the Least Tern (Sterna antillarum). Timing of counts is critical because numbers of fledglings present may be influenced by temporal patterns in fledgling departures from colonies and post-fledging mortality. The objectives of the present study were to measure chick survival and fledgling residence time (FRT) at three Least Tern colonies in Maine and examine the accuracy of single or multiple counts of fledglings for estimating fledgling production and fledging success. Chicks were color-banded at age zero to one d in 2002-2003 to estimate chick survival using mark-resighting methods and to examine relationships between fledging date and FRT. Colony sites were searched for banded chicks and fledglings every one to three d from just prior to fledging until their disappearance from the vicinity of nesting areas at three colonies. Data on numbers of banded fledglings known alive on each survey date were used to simulate single or periodic counts of banded fledglings for the purpose of comparing estimates of fledgling production and success based on count methods with known values. Chick survival estimates were 0.73, 0.74, and 0.14 for the three colonies. Fledgling residence times decreased relative to fledging date at all colonies, and the trend for maximum values of FRT, which may be an indicator of the maximum time to departure, varied from approximately 30 d in mid-July to 15 d in early to mid-August. The highest one-d counts of fledglings underestimated fledgling production by 21-30% for the two sites with high chick survival. Methods based on multiple counts also tended to underestimate fledgling production. Fledgling residency patterns limit the utility of count methods for estimating fledgling production and fledging success.

Wood thrush nest success and post‐fledging survival across a temporal pulse of small mammal abundance in an oak forest

1. Synchronized mass production of seed crops, such as acorns, produces a resource pulse that may have far-reaching consequences for songbird populations through its effects on avian predators. Seed production in these forests represents only the first of several pulsed events. Secondary pulses emerge as mast-consuming rodents numerically respond to seed production and tertiary pulses emerge as generalist predators, such as raptors, numerically respond to rodents. In turn, these two groups reduce nest productivity and juvenile survivorship 1 and 2 years, respectively, after the initial pulse in seed production. 2. At our study site in south-eastern New York, USA, autumn acorn abundance (primary pulse) largely determines rodent abundance (secondary pulse) the following spring. We tested the hypotheses that the population dynamics of a shrub-nesting passerine (wood thrush Hylocichla mustelina), is influenced by rodents through the: (a) direct effect of predation by rodents; (b) indirect effect of rodents on the abundance of raptors (tertiary pulse); and (c) indirect effect of rodent abundance on raptor diet. The latter specifically hypothesizes that a crash in the rodent population in the wake of region-wide failure of acorn production leads to an extreme diet shift in raptors that increases post-fledging mortality in birds. 3. We conducted a 3-year study to examine variation in wood thrush nest success and fledgling survival, using radio telemetry, across a pulse of rodent abundance (i.e. low, medium and high). We also updated and reanalysed regional wood thrush population growth rates as a function of the annual variation in rodent abundance. 4. Fledgling survivorship, but not nest success, varied in relation to annual rodent abundance. Raptors and eastern chipmunks Tamias striatus were the most commonly identified predators on fledglings. Fledgling survivorship was greatest at intermediate rodent abundance consistent with a shift in raptor diet. Regional rate of wood thrush population growth showed a unimodal relationship with rodent abundance, peaking during years with intermediate rodent abundance. This unimodal pattern was due to wood thrush population growth rates near or below zero during rodent population crashes. 5. The telemetry study, pattern of regional abundance and synchronized population dynamics of coexisting thrushes suggest a common mechanism of behavioural changes in raptors in response to declines in rodent prey, which in turn affects thrush population dynamics.

Quantifying density dependence in a bird population using human disturbance

Although density dependence has long been recognised as vital to population regulation, there have been relatively few studies demonstrating it spatially in wildlife populations, often due to the confounding effects of variation in habitat quality. We report on a study of woodlarks Lullula arborea, a species of European conservation concern, breeding on lowland heath in Dorset, England. We take the novel approach of utilising the birds' response to human disturbance, which resulted in much of the variation in density but had no direct impact on demographic rates. Within years, in sites with greater density there were smaller mean chick masses, lower post-fledging survival, and higher rates of nestling mortality attributed to starvation. The effects on clutch size and fledging success were confounded by the area of grassland within a site. There was no effect on brood size. Density dependence also operated within sites between years: as density increased there were reductions in mean chick mass and post-fledging survival, while nestling mortality attributed to starvation increased. Density-dependent effects on clutch size were only weakly regulatory, whereas density-dependent starvation and post-fledging mortality rates contributed strongly to differences in overall breeding output. Heavier chicks (when 7 days old) were significantly more likely to fledge and less likely to starve. Broods with heavier chicks were more likely to supply recruits to the breeding population. Nestling mass was not a factor in survival in the immediate post-fledging period, suggesting that density-dependent processes act independently on this stage. We conclude that the number of birds per hectare of suitable habitat is a valid means of expressing density, and that habitat acts as a surrogate for food abundance through which density dependence operates on the woodlark population.

JUVENILE MORTALITY INCREASES WITH CLUTCH SIZE IN A NEOTROPICAL BIRD

The change in avian clutch size with latitude is a celebrated example of geographic variation in a vertebrate life-history trait. Alternative hypotheses for this pattern invoke nest predation, limited food for nestlings, or post-fledging juvenile mortality as selection pressures leading to small clutch size of tropical birds. We manipulated the clutch size of Spotted Antbirds (Hylophylax naevioides) in central Panama to test these hypotheses. We observed that rates of nest predation were not influenced by parental activity at nests and parents could successfully feed nestlings in enlarged broods. Although larger broods produced the most fledged juveniles, these individuals were less likely to survive to dispersal than were juveniles that fledged from smaller broods. Consequently, nest productivity did not vary with clutch size. Post-fledging mortality was not related to nestling mass two to three days prior to fledging. Rather, differences in the allocation of parental investment per juvenile among differently sized broods appeared to influence juvenile survival probability. These results identify parental care during the post-fledging period as a potential key factor influencing the evolution of small clutch size in tropical birds.

Does fledging equal success? Post-fledging mortality in the Black-fronted Tern

Fledging success is widely used as an index of reproductive success in avian studies, but survival of young after fledging is rarely estimated. In this study, I used radio telemetry to measure rates and causes of mortality of 63 Black-fronted Terns (Sterna albostriata) in the immediate post-fledging period on the Ohau River, South Island, New Zealand, in 1998–2000. Juvenile survival for four weeks after fledging was 0.795. Mean daily survival increased from 0.972 in the first week after fledging to 0.995 in the second week and 1.00 for the third and fourth weeks. Introduced predators such as feral cats (Felis catus) and Norway rats (Rattus norvegicus) were responsible for a minimum of nine out of 12 deaths, and all except one death occurred at the natal colony prior to dispersal. Larger, heavier juveniles that fledged at a younger age tended to have higher survival after fledging, but survival could not be accurately predicted based on measurements of growth. These results indicate that the post-fledging period can be an important period for mortality in young birds, and that survival until dispersal from the natal colony is the best measure of Black-fronted Tern reproductive success currently available.

Post-Fledging Survival of Burrowing Owls in Saskatchewan

Extensive research into the decline of the endangered burrowing owl (Athene cunicularia) has focused on breeding biology, but has largely ignored the post-fledging period. To determine whether post-fledging survival was an important factor in the decline we used radiotelemetry to monitor post-fledging juveniles in a migratory population of burrowing owls in southern Saskatchewan, Canada, during 1997-2000. As part of a concurrent study, 92% of the owl pairs nested in artificial nest boxes, and 64% received supplemental food from hatch until fledging. Survival of juvenile owls during the 3-month, post-fledging period was significantly higher in 1997 (1.00, n = 12) than 1998-2000 (0.55 [95% CI: 0.44 to 0.70], n = 64; P = 0.02). Unusually high survival during 1997 may have been related to the unusually high abundance of voles (Microtus spp.) that year. From 1998 to 2000, 27 of 64 radiomarked owls died. Predators caused the majority of post-fledging mortality (15 of 27), with juveniles at greatest risk the first 14 days after fledging. Other sources of mortality were related to anthropogenic factors (5 of 27) and starvation (5 of 27). Following each of 3 years with moderate juvenile post-fledging survival, the breeding population of burrowing owls declined 11-48% and contained correspondingly few yearling recruits (x = 2.1%). However, following the year of high post-fledging survival (1997), the breeding population increased by 32%, and yearling recruitment was substantially higher (8.3%). These results suggest that post-fledging survival has a considerable influence on population size in the subsequen breeding season, and that management strategies for this species of concern should include this life stage.

MOVEMENTS AND SURVIVAL OF LARK BUNTING FLEDGLINGS

Abstract We quantified post-fledging pre-independence behavior and survival in Lark Buntings (Calamospiza melanocorys) using radio-telemetry. Brood division was recorded in six broods and was maintained throughout the observed fledgling care period. Chicks were capable of short flights (up to 25 m) by fledgling day 6 and longer flights (to 100 m) by fledgling day 13. During the first three weeks after fledging, juveniles moved as far as 800 m from nests. Nine of 23 (39%) monitored fledglings died within 15 days of fledging, primarily due to predation by raptors. Daily survival rates were 0.953 ± 0.019 for fledgling days 0–9, 0.955 ± 0.038 for fledgling days 10–20, and 0.953 ± 0.015 for fledgling days 0–20. The probability of surviving fledgling days 0–20 was 0.367. More quantification of juvenile survival is clearly needed to understand the role of post-fledging mortality in source-sink dynamics. Los Movimientos y Supervivencia de los Volantones de Calamospiza melanocorys Resumen. Cuantificamos la conduct...