Post-fledging Survival Research Articles

Post-fledging recruitment in relation to nestling plasma testosterone and immunocompetence in the spotless starling

1. Social and ecological conditions experienced by individuals during early life can strongly influence their development and survival. Nestlings of many species present important variations in plasma androgens that can be associated with begging and sibling competition and may translate into fitness effects, since broods with higher testosterone (T) production may have better body condition and higher fledging success. However, the positive effects of androgens may be counterbalanced by a reduction of immune defences and a greater susceptibility to diseases. 2. In this study we examined the potential relationships between natural variation in plasma T, immunity and post-fledging survival rate in nestlings of the spotless starling (Sturnus unicolor). 3. We found that nestlings with higher cellular-mediated immune responses (CMI; measured as a swelling response to phytohaemagglutinin injection) were more likely to be recruited in the population than nestlings with lower CMI responses. Males presented higher CMI response than females, possibly due to differences in competitive advantage over food. We also found that CMI response was negatively related to T levels, as predicted by the immunocompetence handicap hypothesis. However, despite this reduction in CMI response, we failed to find an association between nestling T levels and survival prospects. Our results add to the evidence of the role played by immune defences in determining survival prospects in natural populations. 4. In conclusion, our study reveals that CMI response can be considered as a good predictor of post-fledging recruitment. As far as we know, this is the first study attempting to evaluate the relationship between nestling T and post-fledging survival. Our results suggest that the potential benefits accrued by high levels of T in sibling competition during the nestling stage do not translate into increased survival.

Characterizing demographic variation and contributions to population growth rate in a declining population

1. Understanding how demographic variation translates into variation in population growth rate (λ) is central to understanding population dynamics. Such understanding ideally requires knowledge of the mean, variance and covariance among all demographic rates, allowing the potential and realized contribution of each rate to λ to be estimated. Such studies require integrated monitoring of all demographic rates across multiple years and are consequently rare, particularly in declining populations and for species with less tractable life histories. 2. We used 12 years of comprehensive demographic data from a declining ring ouzel (Turdus torquatus) population to estimate the mean, variance and covariance in all major demographic rates and estimate potential and realized demographic contributions to λ. 3. Population size decreased from 39 to 13 breeding pairs (-67%) and mean λ was 0·91 during 1998-2009. This decrease did not reflect a substantial concurrent decrease in any single key demographic rate, but reflected varying combinations of demographic rates that consistently produced λ < 1. 4. Basic prospective elasticity analysis indicated that λ was most sensitive to adult survival, closely followed by early season reproductive success and early brood first-year survival. In contrast, integrated elasticity analysis, accounting for estimated demographic covariance, indicated that λ was most sensitive to early brood first-year survival, closely followed by re-nesting rate, early season reproductive success, late-brood first-year survival and adult survival. 5. Retrospective decomposition of variance suggested that first-year survival contributed most to observed variation in λ. 6. However, demographic comparison with other related species suggested that adult survival, but not reproductive success or post-fledging survival, averaged lower than expected throughout the 12-year study. 7. These data demonstrate that multiple approaches, including comprehensive demographic and comparative analyses and due consideration of conflicting answers, may be necessary to accurately diagnose the demographic basis of population change.

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.

Altitudinal migration in American Dippers (Cinclus mexicanus): Do migrants produce higher quality offspring?

Breeding at high elevations can favour life-history strategies in which parents shift to investing in higher quality rather than higher numbers of offspring. In American Dippers ( Cinclus mexicanus Swainson, 1827), altitudinal migrants produce fewer fledglings than sedentary individuals (residents) that breed at lower elevations. We examined whether migrants compensate for their lower fecundity by providing their offspring with a higher quality diet and (or) more food, and producing higher quality offspring. Nestling diet was assessed using observations and stable isotope analysis of feathers grown during the nestling period. Nestling quality was assessed using a condition index (residuals from a mass–tarsus regression) and postfledging survival. We found that migrants fed their offspring less fish, and despite having higher feeding rates, had lower energetic provisioning rates than residents. Migrants also produced offspring that were in worse condition and had lower postfledging survival. This study found no evidence that altitudinal migration is associated with a trade-off favouring the production of smaller numbers of higher quality young. Instead our data provide support for the hypothesis that altitudinal migration in American Dippers is an outcome of competition for limited nest sites at lower elevations that forces some individuals to move to higher elevations to breed.

Survival benefits of post‐fledging care: experimental approach to a critical part of avian reproductive strategies

1. Caring for offspring beyond leaving the nest is an important but under-studied part of avian life histories. Theory predicts that prolonged post-fledging parental care should yield fitness benefits such as increased fledgling survival. Post-fledging care is also costly in terms of time and energy available for subsequent reproduction, moult or migration. So far, direct measurements of the fitness effects of the duration of post-fledging parental care are lacking. 2. In a partial cross-fostering experiment, barn swallow (Hirundo rustica) chicks were exchanged among broods close to fledging. Thereby, we separated the effects of post-fledging care from those of pre-fledging origin on juvenile survival. 3. Prolonging post-fledging care substantially increased juvenile survival up to 3 weeks post-fledging. Juvenile mortality was maximal in the days following the termination of parental care, and prolonging care delayed and reduced this peak mortality. Survival of fledglings experiencing 6 days of care was Phi = 0.227, whereas fledglings experiencing 14 days of care showed a survival of Phi = 0.571. 4. Offspring from pairs providing short care showed lower post-fledging survival than did offspring from pairs providing long care, irrespective of the actual duration of care experienced. This gives evidence for an additional survival effect of pre-fledging factors associated with the parental duration of care. 5. The results suggest that differential survival in relation to post-fledging parental care is a major fitness component. This relationship has profound effects on the reproductive trade-offs underlying the evolution of avian life histories.

Parental Care, Development of Foraging Skills, and Transition to Independence in the Red-Footed Booby

When and how fledglings become independent are important issues in understanding the life history of long-lived seabirds. In this study, we investigated post-fledging care, development of foraging skills, and the ultimate causes of independence in juvenile Red-footed Boobies (Sula sula) on Dong Island, Xisha Archipelago, northern South China Sea, during 2007 and 2008. After fledging, juveniles took an average of 118 days to become independent; the post-fledging survival rate was 96%. Parents supplied the young with more food after they fledged (average 176 g day-1) than before, but the mass fed per day declined significantly at the end of the post-fledging period. Juveniles' foraging trips lengthened progressively and probably served mainly to develop foraging skills, as 94% of their food was supplied by their parents. Juveniles foraged for themselves mostly from 60 days post-fledging onward, which was just adequate for survival at independence. Fledglings became independent when foraging was energetically more profitable than begging food from the parents, whereas parents significantly reduced the mass of food delivered to the fledgling when the fitness increment from another nesting exceeded that from attending their current young. Nonsystematic checking of the nest area showed that at least 60% of fledglings still survived 20 to 84 days after independence, indicating that the strategy for achieving independence is successful. This study is the first to analyze the independence ontogeny of a long-lived seabird quantitatively.

Effects of Forest Management on Postfledging Survival of Rose-breasted Grosbeaks (Pheucticus ludovicianus)

ABSTRACT. Many studies have examined the effects of forest fragmentation and management on songbird nesting success, but few have quantified postfledging survival, which is a critical component of population productivity. In 2005–2006, we estimated daily postfledging survival of Rose-breasted Grosbeaks (Pheucticus ludovicianus) by radiotracking 42 fledglings in forest fragments that had been managed by single-tree selection, by diameter-limit harvest, or as reference (not harvested for at least 25 years). Survival probability over the 3-week fledgling period was 0.62, and 86% of total fledgling mortality occurred during the first week out of the nest. Despite large differences in forest structure between forest management treatments, there was no effect of forest treatment on fledgling survival. Date of fledging, shrub cover, and patch size also had limited influence on fledgling survival. For all sites combined, females produced an estimated 0.23–0.37 recruiting daughters per year for the worst- and best...

Post-Fledgling Survival of the Cooperatively Breeding Puff-Throated Bulbul (Alophoixus Pallidus)

Abstract. In the tropics, passerines are expected to have survival rates higher than those in temperate zones, though their post-fledging survival has rarely been quantified. Furthermore, because of increased care by adults, cooperative breeding species should have even higher rates of juvenile survival, but few data are available on such species also. Over three breeding seasons we examined factors relating to post-fledging survival of the cooperatively breeding Puff-throated Bulbul (Alophoixus pallidus) in an evergreen forest in northeastern Thailand. These factors included the presence of helpers, fledgling age, sex, body mass prior to fledging, date of fledging within a year, and variation by year. The probability of surviving the 8-week period of dependency was 0.61 ± 0.09 (SE), higher than most reports from the temperate zone. Survival probability was strongly associated with age though not with the presence of helpers. Weekly survival was lowest during the first week post-fledging (0.73 ± 0.08), wh...

Patterns of mortality for each life‐history stage in a population of the endangered New Zealand stitchbird

1. Using data from 396 breeding attempts over an 8-year period, we investigated age- and stage-specific survival rates and their modifying factors in a closed island population of the New Zealand stitchbird (or hihi, Notiomystis cincta Du Bus). 2. Survival probability generally increased over time; however, at each life-history transition, survival in the new stage started lower than at the end of the previous stage, creating a 'saw-tooth' function of age-related survival. 3. The probability of an egg hatching was low (0.73 +/- 0.01): most likely a consequence of genetic bottlenecks previously endured by this population. There was strong support for a positive relationship between hatching rate and the subsequent survival of the female parent, and hatching success declining for females > 4 years old. 4. Nestling survival probability increased as a function of brood size and days since hatching, and decreased relative to daily maximum ambient temperature and hatching date. Support for models including ambient temperature was greater than for other covariates, with the majority of this temperature-mediated survival effect being restricted to the early nestling stage. 5. Fledglings had low survival rates in the first two weeks after leaving the nest, with post-fledging survival related to the fledgling's mass. Two months after fledging, juvenile survival probability plateaued and remained relatively constant for the following autumn, winter and spring/summer breeding season. There was no effect of sex or season on adult survival probability. However, there was strong support for age-specific variation in adult survival, with survival likelihood increasing during the first four years before showing evidence of a senescence decline. 6. Within-stage survival increases were likely related to stage-specific selection pressures initially weeding out individuals of poorer phenotypes for the environment specific to each life-history stage. Such a mechanism explains the initial high mortality at life-history transitions; a well-adapted phenotype for one stage may not necessarily be so well adapted for subsequent stages. These patterns are not only valuable for examining life-history theory, but also for understanding the regulation of vital rates in an endangered species and providing a basis from which better population management models and harvesting regimes can be derived.

Fine‐scale life‐history variation in sociable weavers in relation to colony size

1. This study investigates the mechanistic and ecological basis of phenotypic sorting in the sociable weaver Philetairus socius (Latham). In this colonially and communally breeding African bird, morphology, condition, age-specific survival rates and reproductive investment vary with colony size. 2. This variation might arise from non-adaptive constraints imposed by density dependence, or from adaptive life-history responses to colony size-dependent selection pressures. To attempt to distinguish these, the environments in which adults made reproductive decisions and in which offspring developed were manipulated. 3. When food supplementation improved the pre-breeding environment of adults, they bred earlier but did not change investment in eggs or incubation. 4. Nestling and subsequent adult phenotypes were unaffected by supplementary feeding, although post-fledging survival improved. Nestling origin and not rearing environment predicted phenotype in a cross-fostering experiment between colonies. 5. Phenotypic differences among colonies hence seemed not to be plastic responses to resource availability. Life-history differentiation, mediated either genetically or maternally, might have taken place over relatively narrow temporal and spatial scales. 6. Adaptive variation in reproductive investment could thus mitigate costs of large colony size, by reducing costs of reproduction and hence improving adult survival. If so, then fine-scale life-history adjustment could help to maintain stable variation in colony size.

Testing the efficacy of a boundary fence at an important tropical seabird breeding colony and key tourist destination

Tourists have the potential to detrimentally impact breeding seabirds, particularly at popular destinations such as on the Great Barrier Reef. Michaelmas Cay is a significant seabird rookery and prime tourist destination on the reef. In 1990, Queensland Parks &amp; Wildlife Service erected a fence to separate tourists from fragile nesting habitat. We used two independent assessments during different breeding seasons to determine the potential impacts of tourism on two pelagic seabird species breeding on the cay. In the first quasi-experiment, egg losses by sooty terns (Sterna fuscata) and common noddies (Anous stolidus) were monitored at four distances (3, 6, 18 and 36 m) from a tourist enclosure. Our second quasi-experiment involved monitoring adult provisioning rates, chick growth and chick survival of sooty terns at two locations, one adjacent to the tourist fence and one 50 m away. At plots 3–6 m from the fence, we observed higher among-week variation in egg loss but no differences in total egg loss as compared with the more distant plots. The only difference observed between plots during our second quasi-experiment was that nest predation was higher at the tourist fence plot. Our research suggests that as long as the effects observed do not influence post-fledging survival or gull predation does not impact under different conditions, current management protocols are appropriately facilitating shared usage between wildlife and the tourism industry at Michaelmas Cay.

FITNESS CONSEQUENCES OF PRE- AND POST-FLEDGING TIMING DECISIONS IN A DOUBLE-BROODED PASSERINE

The fitness consequences of a delayed timing of breeding are expected to affect the temporal characteristics of the whole annual breeding system. One major problem in quantifying the fitness relevance of timing is that individual differences between pairs may cause the seasonal trend. Differentials in juvenile survival due to pre-fledging timing decisions often only appear after fledging of the chicks. Therefore, timing decisions in the post-fledging period, i.e., the duration of parental care, might additionally influence juvenile survival. We tested the effects of timing and parental competence on the post-fledging survival of second-brood juvenile Barn Swallows (Hirundo rustica L.) by swapping earlier and later hatching clutches and radio-tracking the juvenile subjects. The mark-recapture models controlled for the effects of duration of post-fledging care and food availability. There was an annually varying negative seasonal trend in offspring survival that was associated with environmental conditions. Directional selection for early breeding occurred in the two years with scarce autumnal food supply. Furthermore, we found strong selection for long post-fledging parental care. The duration of care neither declined seasonally, nor did longer care compensate for the seasonal decline of juvenile survival. Hence, the reproductive output three weeks after fledging was determined by two parental timing decisions: the timing of breeding and the timing of family breakup. We suggest that differential survival of second-brood fledglings in relation to these decisions is an important part of the selective mechanisms shaping the reproductive system of Barn Swallows.

Ectoparasites affect hemoglobin and percentages of immature erythrocytes but not hematocrit in nestling Eastern Bluebirds

Abstract Populations of the hematophagous mite, Dermanyssus prognephilus, within Eastern Bluebird (Sialia sialis) nests were manipulated using a pyrethrin-based insecticide to examine effects on nestling growth, selected hematological parameters, and fledging success. There was no difference in body mass, fledging success, estimated white blood cell counts, or hematocrits of infested versus non-infested nestlings. However, infested nestlings had a 25.0% reduction in hemoglobin and a 20.5% increase in immature erythrocytes. These findings suggest a physiological response to parasite feeding that may affect post-fledging survival.

Survival of postfledging Forster’s terns in relation to mercury exposure in San Francisco Bay

We examined factors influencing mercury concentrations in 90 fledgling Forster's terns (Sterna forsteri) and evaluated whether mercury influenced postfledging survival in San Francisco Bay, California. Mercury concentrations (+/- SE) in chicks 21-29 days old (just before fledging) were 0.33 +/- 0.01 microg g(-1) ww for blood and 6.44 +/- 0.28 microg g(-1) fw for breast feathers. Colony site had an overriding influence on fledgling contamination, however hatching date and age also affected blood, but not feather, mercury concentrations. Blood mercury concentrations decreased by 28% during the 50-day hatching period and increased with chick age by 30% during the last week prior to fledging. Using radio-telemetry, we calculated that cumulative survival during the 35-day postfledging time period was 0.81 +/- 0.09 (SE). Postfledging survival rates increased with size-adjusted mass, and cumulative survival probability was 61% lower for terns with the lowest, compared to the highest, observed masses. Conversely, survival was not influenced by blood mercury concentration, time since fledging, sex, or hatch date. Mercury concentrations in breast feathers of fledglings found dead at nesting colonies also were no different than those in live chicks. Our results indicate that colony site, hatching date, and age influenced mercury concentrations in fledgling Forster's terns, but that mercury did not influence postfledging survival.

Prevalence of avian pox virus and effect on the fledging success of Laysan Albatross

Avian pox virus (Poxvirus avium) is a mosquito-borne disease that occurs worldwide in a variety of bird species, but little is known about its prevalence or effect on seabirds. We monitored prevalence of pox virus and its effect on fledging success of Laysan Albatross (Phoebastria immutabilis) on Oahu, Hawaii, from 2003 to 2007. Pox prevalence in albatross chicks averaged 88% in years with high rainfall and 3% in years with low rainfall. Diagnosis of pox virus was clinically confirmed in two birds by Muscovy Duck (Cairina moschata) fibrolast cultures. Severity of infection ranged from small wart-like nodules and lesions on the bill, face, eyes, tarsus, and feet, to large tumorous growths that completely covered both eyes and caused deformation of the bill and skull. Most chicks recovered from infection, and the fledging rate in pox epizootic years (82%) did not differ from that in years with low pox prevalence (80%) or the average fledging rate on Midway Atoll (86%). Three chicks with severe infections were resighted as healthy adults on Kauai and Oahu in 2007, confirming postfledging survival of at least some birds. The high recovery rate, fledging success, and postfledging survival indicate that Laysan Albatross have strong immunity to avian pox virus.

Recent Advances in the Behavioral Ecology of Tropical Birds

Tropical birds offer unique opportunities to test ecological and evolutionary theory because their life history traits are so diverse and different from temperate zone models upon which most empirical studies are based. We review recent studies on the behavioral ecology of tropical birds, studies that explore new advances in this field. Life histories and their evolution remain the focus of research on tropical birds. Clutch size manipulations in two species showed that food limitation does not explain small clutch size. In antbirds, enlarged clutches decreased post-fledging survival whereas in thrushes, enlarged broods were costly due to high nest predation. Small clutches may be favored via different ultimate selective forces and shared underlying tradeoffs between the immune, metabolic, and endocrine systems in the body may account for the commonly observed ‘slow pace of life’ in tropical birds. The physiological tradeoff between testosterone and immunocompetence may explain the evolution of low testosterone levels in tropical passerines where adult survival is paramount. In contrast to life history theory, few studies have explored temperate-tropical differences in territoriality, mating systems, and song function. The idea that low breeding synchrony in tropical birds is associated with low levels of extra-pair fertilizations was supported by several new paternity studies conducted on tropical passerines. Seasonally breeding tropical birds have higher testosterone levels than tropical birds with prolonged breeding seasons, although it is unclear if this pattern is driven by mating systems per se or selection from pathogens. Recent work on relations between pair members in permanently paired tropical passerines focuses on the question of mate defense versus territorial defense and the extent of cooperation versus selfish interests in inter-sexual relations.

Influence of temperatures during the nestling period on post-fledging survival of great tit Parus major in a Mediterranean habitat

Influence of temperatures during the nestling period on post-fledging survival of great tit Parus major in a Mediterranean habitat

Mercury and drought along the lower Carson River, Nevada: II. Snowy egret and black-crowned night-heron reproduction on Lahontan Reservoir, 1997–2006

Mercury concentrations in the floodplain of the Carson River Basin in northwestern Nevada are some of the highest ever reported in a natural system. Thus, a portion of the basin including Lahontan Reservoir was placed on the U.S. Environmental Protection Agency's Natural Priorities List for research and cleanup. Preliminary studies indicated that reproduction in piscivorous birds may be at risk. Therefore, a 10-year study (1997--2006) was conducted to evaluate reproduction of snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) nesting on Gull Island in Lahontan Reservoir. Special attention was given to the annual flow of the Carson River, the resultant fluctuation of this irrigation reservoir, and the annual exposure of snowy egrets and night-herons to methylmercury (MeHg). The dynamic character of the river due to flooding and drought (drought effect) influenced snowy egret and night-heron reproduction more so than did MeHg contamination of eggs. During an extended drought (2000--2004) in the middle of the study, snowy egret nests containing eggs with concentrations of MeHg (measured as total mercury [THg] approximately 100% MeHg) > or =0.80 microg THg/g, ww, all failed, but in 1997 and 2006 (wet years with general flooding), substantial numbers of young were produced (but fewer than at nests where eggs contained <0.80 microg/g). Thus, a variable reproductive threshold of tolerance to MeHg may be associated with habitat quality (food type and abundance). Clearly, drought was the most important factor affecting snowy egret annual productivity. In contrast to snowy egrets, night-herons generally had fewer nests meeting the 0.80 microg THg/g criterion, and those above the criterion were less sensitive to mercury than were snowy egrets. Furthermore, night-herons appeared more tolerant of drought conditions than snowy egrets because they nested earlier, selected more protected nesting sites, and had a more generalist diet that provided additional food options including terrestrial organisms, which also reduced exposure to MeHg. A putative biological effect threshold of 2.0 microg THg/g in whole blood for young of both species was evaluated, which was frequently exceeded, but with no evidence, while still in the colony, of an association with direct mortality. An evaluation of physiological associations with blood residues and post-fledging survival will be presented in future reports in this series.

Survival of Postfledging Grassland Birds in Missouri

Abstract We describe survival patterns during the postfledging period for two species of grassland birds in Missouri. We monitored 155 radio-marked juvenile Dickcissels (Spiza americana) and 107 juvenile Eastern Meadowlarks (Sturnella magna; hereafter meadowlark) in southwestern Missouri, from 2002 to 2004. We used an information theoretic approach to evaluate support for biological, temporal, and spatial covariates on survival estimates using Cox proportional hazards models, and also identified cause-specific mortality. For Dickcissels, the model with biological covariates containing body mass had the lowest score using Akaike's information criterion, corrected for small sample sizes (AICc) and almost twice the support of the second-best model. For meadowlarks, the null model had the lowest AICc score, but the second-best model containing body mass was also competitive (within 2 AICc units), so we used the latter model for inference. Hazard ratios indicated that a 1 g increase in body mass was associated with a 3% (Dickcissels) and 2% (meadowlarks) reduction in the risk of death. The cumulative probability of surviving the study period (Dickcissels, 58 days, meadowlarks, 72 days) was 0.56 (95% CI: 0.49–0.65) for Dickcissels and 0.63 (95% CI: 0.54–0.74) for meadowlarks. Predation was the largest source of cause-specific mortality for both species and responsible for 56% (Dickcissels) and 70% (meadowlarks) of deaths. Snakes were the numerically dominant predator of juvenile Dickcissels, but there was not a numerically dominant predator of juvenile meadowlarks. Our results emphasize the importance of body condition on postfledging survival. Future studies should consider estimating postfledging survival rates within species across systems to understand the potential impacts of different predator communities on juvenile survival.

Increased Thyroid Hormone Levels in Tree Swallows (Tachycineta bicolor) on Reclaimed Wetlands of the Athabasca Oil Sands

The oil sands of Alberta, Canada are one of the world's largest reserves of crude oil. Oil sands mining companies are now investigating the ecological impacts of reclamation strategies in which wetlands are used for the bioremediation of waste materials. To examine the endocrine disrupting potential of chemicals in Oil Sands Process Materials (OSPM), thyroid hormone concentrations were measured in plasma and thyroid glands of nestling tree swallows (Tachycineta bicolor) from wetlands partly filled with mine tailings. Plasma triiodothyronine (T(3)) concentrations and thyroxine (T(4)) content within thyroid glands were elevated in nestlings from OSPM sites compared to those from the reference site. Results suggested enhanced hormone synthesis by the thyroid glands independently of activation of the pituitary-thyroid axis, as well as increased deiodination of T(4) into T(3) in peripheral tissues. This might have resulted from exposure to oil sands associated chemicals such as polycyclic aromatic hydrocarbons and from environmental factors such as food availability. Modulation of thyroid function might have negative effects on metabolism, behavior, feather development, and molt, which could compromise postfledging survival.