An assay to investigate factors influencing initial orientation in nocturnally fledging seabirds

The first solitary migration of juvenile birds is difficult to study because of a low juvenile survival rates and sometimes long delays in return to the breeding grounds. Consequently, little is known about this crucial life event for many bird species, in particular the sensory guidance mechanisms facilitating the first migratory journey. Initial orientation during the first migration is a key measure to investigate these mechanisms. Here, we developed an assay to measure initial orientation as flight direction upon first take‐off in nocturnally fledging juvenile seabirds. We dorsally deployed a coloured LED on juvenile birds to allow researchers to observe the vanishing bearings of individuals as they flew out to sea. Additionally, we co‐deployed either a small Neodymium magnet or glass bead (control) on top of the bird's head to investigate the use of magnetoreception, previously unexplored in this early life stage. We used this assay to observe the first flight of Manx shearwaters Puffinus puffinus and found that they did not orient towards their wintering ground straight after taking off. Further, we did not find an effect of the magnetic treatment on juveniles' flight direction, though whether this is due to the birds not using magnetoreception, other salient cues being available or a lack of motivation to orient to the migratory beeline is unclear. We were, however, able to identify wind direction and topography as drivers of first flight direction in Manx shearwaters, which fledged with wind component between a crosswind and a tailwind and directed their maiden flight towards the sea and away from the land. This novel assay will facilitate the study of the maiden flight of nocturnally fledging birds and will help advance the study of sensory guidance mechanisms underpinning migratory orientation in a wide range of taxa, including species which are traditionally challenging to study.     

Is the juvenal plumage of altricial songbirds an honest signal of age? Evidence from a comparative study of thrushes (Passeriformes: Turdidae)

The function and evolution of avian plumage colouration has been the subject of many studies over the past decade, but virtually all of this research has focused on the plumages of sexually mature individuals. The colours and patterns of juvenal plumage, which is worn by altricial songbirds only for the first few months of life, have been the focus of few studies. We develop the idea that distinctive juvenile appearance may be a signal of sexual immaturity, serving to reduce aggression from conspecific adults. We use a comparative phylogenetic approach to test this hypothesis in the thrushes (Family Turdidae). Honest signals of reproductive immaturity should be more valuable when juveniles fledge into environments with aggressive adult conspecifics. Therefore, we predicted that distinctive juvenile appearance would be more likely to evolve in species with extended breeding seasons and high levels of territoriality. Because many tropical bird species exhibit year‐round territoriality and elongated breeding seasons, we used breeding latitude as a proxy for these variables. As predicted, distinctive juvenile appearance was significantly correlated with occupancy of tropical latitudes. While alternative explanations cannot be ruled out and more tests of the hypothesis are needed, the observed associations between breeding latitude and distinctiveness of juvenal plumage are consistent with our hypothesis that distinctive juvenal plumage evolved as a signal of sexual immaturity.     

Development and maturation of plumage in the wandering albatross Diomedea exulans

Changes in the plumage of the wandering albatross Diomedea exulans were studied on the Crozet Islands, using a population of birds of known sex and age and including some birds more than 32 years old. Plumage phases of a cross-section of the male and female populations are presented. Males and females fledge with a dark brown plumage. Between the first and fourth year the male's plumage becomes much whiter than the female's. Between five and 15–18 years old the whitening of plumage in either sex develops in parallel though still separated by the extent of the initial divergence. Male plumage probably attains a definitive snowy stage after 30 years while the plumage of the female does not mature beyond an intermediate stage, which is reached after 20 to 25 years. Maturation of the plumage of head, back and wing are compared. In birds of similar age, breeding birds tend to have a whiter plumage than non-breeders. In the oceanic range of the species, white birds, i.e. mostly adult males, favour cold antarctic waters while dark birds, i.e. mostly adult females and juveniles, are observed in warmer subtropical and subantarctic waters. We discuss the possible adaptive significance of the slow maturation in the plumage of the wandering albatross and of the differences in plumage between sexes and between populations.     

Unusual feather structure allows partial plumage wettability in diving great cormorants Phalacrocorax carbo

The great cormorant Phalacrocorax carbo is thought to have a wettable plumage, providing low body insulation during foraging. Great cormorants should thus be constrained by water temperatures, and show high energy requirements. Surprisingly, this species has one of the widest breeding distributions of all diving birds, and does not require more food than these other species. We explored this apparent paradox by comparing the insulative properties of body plumage in four subspecies of great cormorants ranging from tropical to polar regions. We found that all subspecies retained an insulating air layer in their plumage, which was, however, much thinner than for other species of diving birds. Detailed examination of the plumage showed that each cormorant body feather has a loose, instantaneously wet, outer section and a highly waterproof central portion. This indicates that the plumage of great cormorants is only partly wettable, and that birds maintain a thin layer of air in their plumage. Our findings suggest an unusual morphological-functional adaptation to diving which balances the antagonist constraints of thermoregulation and buoyancy.     

Takeoff flight performance and plumage wettability in Cassin’s Auklet Ptychoramphus aleuticus, Xantus’s Murrelet Synthliboramphus hypoleucus and Leach’s Storm-petrel Oceanodroma leucorhoa

Due to their marine habitats and distinctive foraging modes, seabirds face unique challenges with respect to flying that are negotiated differently by various species. One such challenge is taking off from the water with wet plumage. This study evaluated plumage wettability and takeoff performance in three seabird species: two wing-propelled divers with high wing loading, Cassin’s Auklet Ptychoramphus aleuticus and Xantus’s Murrelet Synthliboramphus hypoleucus; and Leach’s Storm-petrel Oceanodroma leucorhoa, a surface feeder with low wing loading. The plumages of the diving birds held less water than that of O. leucorhoa (~6.7% of body mass vs 9.5%). This difference is explained by O. leucorhoa’s surface to volume ratio being larger than that of the alcids. Furthermore, the alcids have afterfeathers larger than those of O. leucorhoa, which promotes a better insulation during diving. Examination of takeoff performance both before and after experimentally submerging the birds indicated that wingbeat frequency, speed and mass-specific power (peak and mean), and energy per wingbeat decreased in all species when plumage was experimentally wetted, whereas mean acceleration increased. O. leucorhoa was more strongly affected by wet plumage than the alcids, with a 32% of reduction in mass-specific energy per wingbeat compared to ≤25% in the alcids. Takeoff angle was reduced in alcids, but not significantly so in O. leucorhoa. Our results offer insights into the takeoff mechanics problems of wet seabirds given their differences in life history and morphology.     

Sources of distinctness of juvenile plumage in Western Palearctic passerines

Juveniles of many avian species possess a spotted or mottled body plumage that is visually distinct from the plumage of adults. In other species, however, juveniles fledge with a body plumage that is just a pale representation of adult female plumage. The reasons for this variation are poorly understood. Several hypotheses concerning social (parent–offspring, adult–juvenile, juvenile–juvenile), ecological (predation risk) and physiological (costs of plumage development) implications of juvenile body plumage are presented in relation to predictions concerning associations with certain ecological and life‐history attributes of avian species. In the present study, we conduct a phylogenetically corrected comparative analysis of Western Palearctic passerines looking for sources of variation in the incidence of distinct and adult‐like juvenile body plumages. We scored plumages based on plates in the Handbook of the Birds of the Western Palearctic (Cramp & Perrins, 1988–1994; Oxford University Press) (HBWP) and entered body mass, migratory habits, habitat, nestling diet, breeding dispersion, gregariousness, duration of the nestling period, type of nest, conspicuousness of female plumage, and sexual dimorphism as explanatory variables, as presented in HBWP, in phylogenetic generalized least square regression analyses. One‐third of the species presented distinct juvenile body plumages, which lasted on average for the first 2 months of life. Body mass, conspicuousness of female plumage, migratory habits, and habitat were significantly associated with interspecific variation in distinctness of juvenile plumage, with smaller species, more conspicuous species, migrants, and species from forested habitats showing distinct juvenile plumages with higher frequency. The phylogenetic signal was moderately high. Assuming that conspicuous adult plumage is costlier to produce than distinct juvenile body plumage (pigments, conspicuousness), the need to acquire social status among juveniles before the winter may explain the more adult‐like plumage in resident species because juveniles will probably compete with individuals that they may have known during their first months of life. On the other hand, migrant juveniles may compete with a different set of individuals in winter quarters and can use savings in resources necessary for developing adult‐like plumages to improve migration capacity by allocating resources to other functions. The association with habitat could be related to juveniles in open habitats participating in more extended interactions with other juveniles than in forested habitats where lower visibility may reduce the capacity to detect or respond to signals from juvenile conspecifics. More studies on this possibly crucial life stage are needed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 440–454.     

Elevated metabolic costs while resting on water in a surface feeder: the Black-legged Kittiwake Rissa tridactyla

Measurements of the energy costs of individual behaviours provide insights into how animals trade-off resource allocation and energy acquisition decisions. The energetic costs while resting on water are poorly known for seabirds but could comprise a substantial proportion of their daily energy expenditure. We measured the cost of resting on water in Black-legged Kittiwakes Rissa tridactyla , a species which does not fly during the night and for which estimating energy expenditure while resting on the water is therefore important. Their resting metabolic rate on water at 12.5 °C was at least 40% higher compared with resting at the same temperature in air. This indicates that, at comparable temperatures, metabolic costs are elevated for birds resting at sea compared with on land. We argue that Kittiwakes meet much of this extra thermoregulatory demand by dedicated metabolic activity. During the winter months, their costs are likely to be even higher owing to lower sea temperatures. Accordingly, we suggest that migration to milder latitudes, following breeding, will provide enhanced benefits, particularly to seabirds such as Kittiwakes which rest on the sea surface during darkness.     

A fine-scale time budget of Cape gannets provides insights into the foraging strategies of coastal seabirds

Central-place foragers organize their feeding trips both to feed themselves and to provide their offspring with food. In seabirds, several long-range foragers have been shown to alternate long and short trips to balance these dual needs. However, the strategies of short-range foragers remain poorly understood. We used a precise, miniaturized motion sensor to examine the time budget of 20 breeding Cape gannets, Morus capensis, foraging off the coast of South Africa. Birds stayed at sea for 5.5-25.3 h, occasionally spending the night at sea. The large number of isolated dives and extended flight time observed during these overnight trips suggested that birds either experienced poor foraging conditions or exploited more distant, yet more profitable prey patches. Conversely, birds that stayed at sea for less than 1 day had relatively consistent activity patterns. Most of these birds (88%) foraged actively at the beginning and at the end of the foraging trip. These feeding bouts were separated by protracted periods of sitting on the sea surface. Such resting periods probably allow birds to digest the food ingested during the first part of the foraging trip, so they initially feed themselves, and then obtain food for their chick on the way back to the breeding site.     

Age-related differences in plumage characteristics of male tree swallows Tachycineta bicolor: hue and brightness signal different aspects of individual quality

Age-related differences in plumage characteristics of birds can be the result of differential survival of more ornamented individuals, within-individual changes in plumage attributes with age, or a combination of both. In this study, we investigated age-class related differences in plumage attributes of male tree swallows Tachycineta bicolor by performing both cross-sectional and longitudinal analyses. Male tree swallows in their first breeding season do not display delayed plumage maturation, and possess, like experienced breeders, metallic green to metallic blue iridescent plumage on their dorsal surface. Our results showed that, at the population level, older males were brighter and reflected light maximally at shorter wavelengths (i.e. were more blue). Differences in plumage brightness were most likely caused by changes within individuals as males increased in brightness between the first time they were captured and the subsequent year. Differences in hue, however, were not due to within-individual changes, but rather appear to be the result of greener individuals having lower survival and/or nest site fidelity. Indeed, relatively dull, greener birds had a lower probability of being recaptured the subsequent year. In contrast, we found that if birds captured in their first year as breeding adults were relatively bright, hue was not related to the probability of recapture. These results suggest that plumage attributes in male tree swallows have the potential of being honest signals of quality. Furthermore, plumage brightness and plumage hue might signal different aspects of male quality in this species.     

Juvenile coloration of Florida Scrub-Jays ( Aphelocoma coerulescens ) is sexually dichromatic and correlated with condition

The Florida Scrub-Jay is a monogamous cooperative breeder in which both males and females display extensive structurally based blue plumage. Juveniles of this species exhibit blue tail and wing feathers that they begin growing as nestlings, and some of these feathers are retained throughout their first year. Although the birds appear to be sexually monochromatic, we assessed whether cryptic dichromatism exists in both the magnitude and pattern of coloration in tail feathers of juvenile Florida Scrub-Jays. We then determined whether variation in plumage coloration is associated with nutritional condition during molt. Tails of juvenile male Florida Scrub-Jays exhibit a greater proportion of UV reflectance than those of females. Mass at age 11 days and ptilochronology of the juvenile tail feathers were used as measures of individual nutritional condition during feather growth, and the latter was found to be positively associated with UV chroma. These data demonstrate that Florida Scrub-Jays are sexually dichromatic and suggest that variation in plumage color may be condition dependent, although we cannot rule out alternative explanations. Juvenile plumage coloration, therefore, has the potential to function as a signal of individual quality in both males and females.     

Rhinoceros Auklet pair‐mates migrate independently but synchronize their foraging activity during the pre‐laying period

Pair bonds are considered important for successful breeding in monogamous birds but their maintenance may be challenging for migratory species, as mates can be separated for months during the non‐breeding period. To investigate whether mates of monogamous migratory seabirds stay together throughout the non‐breeding period and how and when they start synchronizing their activity before breeding, we tracked seven pairs and 22 individuals of Rhinoceros Auklets Cerorhinca monocerata with geolocators and saltwater immersion loggers. Mates migrated across similar areas during the non‐breeding period but with a sustained temporal shift, putting them an average of 377 km apart and resulting in an average difference of return date at the colony of 5.6 days, with no sex biases. These values did not differ significantly from those between ‘pairs’ of randomly selected, non‐mated birds. Mates showed synchronized on‐water/in‐air at‐sea activities once both birds returned and spent the first night together at the colony. The synchronization of activities was highest on the day following the nights when both mates visited the colony, and decreased with elapsed time. Mates then left the colony together for a pre‐laying exodus of 8–9 days and males returned 2–4 days earlier than females before incubation started. Mates kept synchronizing at‐sea activity during the early part of the exodus. We interpret this as the mates staying together at sea during the pre‐laying period, increasing the males’ chances of copulation at sea. The patterns of mate association observed in Rhinoceros Auklets contrast with those of the Procellariiformes, presumably reflecting differences in the place and timing of copulation.     

Postnatal dispersal of wandering albatrosses Diomedea exulans: implications for the conservation of the species

Many large marine vertebrates are today threatened by human activities and it is therefore crucial to obtain information on their distribution and behaviour at sea. In particular little is known about the time necessary for juveniles to acquire the foraging skills of adults. We tracked 13 juvenile wandering albatrosses Diomedea exulans by satellite telemetry during their first year at sea. They covered an average distance of 184,000 km during the first year and restricted their dispersal to the unproductive waters of the subtropical Indian Ocean and Tasman Sea. This region of low wind velocities does not overlap with the foraging areas used by adults. After an innate phase of rapid dispersal with a fixed flight direction, young birds progressively increased their daily flight distances and attained adult flight efficiency within their first six months at sea. The complete overlap of the juveniles' foraging ranges with major long-line fisheries in the subtropical waters constitutes a major threat that could jeopardize the long term recovery ability of populations of the endangered wandering albatross in the Indian Ocean.     

Postnatal dispersal of wandering albatrosses Diomedea exulans: implications for the conservation of the species

Many large marine vertebrates are today threatened by human activities and it is therefore crucial to obtain information on their distribution and behaviour at sea. In particular little is known about the time necessary for juveniles to acquire the foraging skills of adults. We tracked 13 juvenile wandering albatrosses Diomedea exulans by satellite telemetry during their first year at sea. They covered an average distance of 184,000 km during the first year and restricted their dispersal to the unproductive waters of the subtropical Indian Ocean and Tasman Sea. This region of low wind velocities does not overlap with the foraging areas used by adults. After an innate phase of rapid dispersal with a fixed flight direction, young birds progressively increased their daily flight distances and attained adult flight efficiency within their first six months at sea. The complete overlap of the juveniles’ foraging ranges with major long‐line fisheries in the subtropical waters constitutes a major threat that could jeopardize the long term recovery ability of populations of the endangered wandering albatross in the Indian Ocean.     

Delayed plumage maturation increases overwinter survival in North Island robins

Many bird species show delayed plumage maturation (DPM), retaining sub-adult plumage until after their first breeding season. Most explanations assume that DPM increases fitness over the breeding season. However, unless birds undergo a full moult before breeding, DPM could also be an adaptation to increase survival over the previous winter. The winter adaptation hypothesis has never been tested owing to the difficulty of measuring overwinter survival. We experimentally tested this hypothesis in North Island robins (Petroica longipes) using a closed island population where we could accurately estimate survival. The experiment involved dyeing 41 juveniles to mimic adult males, and comparing their survival with 41 control juveniles treated with the same peroxide base minus the pigment. The population was monitored with a series of resighting surveys, and mark-recapture analysis used to estimate overwinter survival. Survival probability was estimated to be 10% for dyed birds versus 61% for control birds in 2001, and 29% for dyed birds versus 40% for control birds in the winter of 2002, supporting the winter adaptation hypothesis for DPM. Access to suitable habitat is the key factor limiting juvenile survival in this population, and the locations where dyed juveniles were sighted suggest that they were often excluded from suitable areas.     

Sensitivity to dimethyl sulphide suggests a mechanism for olfactory navigation by seabirds

Petrels, albatrosses and other procellariiform seabirds have an excellent sense of smell, and routinely navigate over the world's oceans by mechanisms that are not well understood. These birds travel thousands of kilometres to forage on ephemeral prey patches at variable locations, yet they can quickly and efficiently find their way back to their nests on remote islands to provision chicks, even with magnetic senses experimentally disrupted. Over the seemingly featureless ocean environment, local emissions of scents released by phytoplankton reflect bathymetric features such as shelf breaks and seamounts. These features suggest an odour landscape that may provide birds with orientation cues. We have previously shown that concentrated experimental deployments of one such compound, dimethyl sulphide (DMS), attracts procellariiforms at sea, suggesting that some species can use it as a foraging cue. Here we present the first physiological demonstration that an Antarctic seabird can detect DMS at biogenic levels. We further show that birds can use DMS as an orientation cue in a non-foraging context within a concentration range that they might naturally encounter over the ocean.     

ISOLATING MECHANISMS IN SEABIRDS

In most species of birds, differences in plumage coloration or song structure act as isolating mechanisms. In seabirds, plumages are generally drab, and vocal repertoires are limited so that other phenotypic attributes must act as isolating mechanisms. One classic study of gulls suggests that the contrast between eye color and head color acts as a reproductive isolating mechanism, but this idea has been largely refuted by widespread evidence of hybridization in gulls in the last 20 years. An examination of more than 100 species of seabirds that breed sympatrically with congeners reveals that species with bills and feet similar in color hybridize in all areas where they breed sympatrically. Species that have bills or feet of different colors either do not hybridize or they produce rare hybrids that are unable to obtain mates as adults. This suggests that bill and foot coloration act as the primary isolating mechanisms in all surface-nesting seabirds and some burrow nesters. It may be a general pattern in birds that foot color acts to supplement bill color in reproductive isolation and mate choice.     

Testosterone increases display behaviors but does not stimulate growth of adult plumage in male golden-collared manakins (Manacus vitellinus)

In order to attract females, male golden-collared manakins gather in leks and perform a complex display consisting of acrobatics accompanied by loud "wingsnapping". During this display, males show off their yellow beard and yellow, black, and green plumage that is striking in comparison to the dull green plumage of young males and females. We investigated the role of testosterone (T) in activating the display of manakins and in stimulating the growth of the adult male plumage. T regulates song, copulation, and territorial aggression in temperate species. In tropical species, however, T levels can be relatively low year round, which has raised questions about the involvement of T in courtship display and male aggression in these species. In neither temperate nor tropical species has the role of hormones in the shift from juvenile to adult plumage been well studied. Therefore, we implanted green-plumaged birds and adult males with either a T pellet or an inert pellet (controls) and observed the display behaviors of these birds in the field and in captivity. In captive birds, we also plucked feathers from sexually dimorphic regions and observed color and regeneration rate of new feathers. We found that birds implanted with T increased several display behaviors compared to controls. All plucked feathers grew back the same color as prior to treatment; however, we observed some differences in feather growth rate between T-treated birds and controls.     

Integrated population modeling reveals the impact of climate on the survival of juvenile emperor penguins

Early‐life demographic traits are poorly known, impeding our understanding of population processes and sensitivity to climate change. Survival of immature individuals is a critical component of population dynamics and recruitment in particular. However, obtaining reliable estimates of juvenile survival (i.e., from independence to first year) remains challenging, as immatures are often difficult to observe and to monitor individually in the field. This is particularly acute for seabirds, in which juveniles stay at sea and remain undetectable for several years. In this work, we developed a Bayesian integrated population model to estimate the juvenile survival of emperor penguins (Aptenodytes forsteri), and other demographic parameters including adult survival and fecundity of the species. Using this statistical method, we simultaneously analyzed capture–recapture data of adults, the annual number of breeding females, and the number of fledglings of emperor penguins collected at Dumont d'Urville, Antarctica, for the period 1971–1998. We also assessed how climate covariates known to affect the species foraging habitats and prey [southern annular mode (SAM), sea ice concentration (SIC)] affect juvenile survival. Our analyses revealed that there was a strong evidence for the positive effect of SAM during the rearing period (SAMR) on juvenile survival. Our findings suggest that this large‐scale climate index affects juvenile emperor penguins body condition and survival through its influence on wind patterns, fast ice extent, and distance to open water. Estimating the influence of environmental covariates on juvenile survival is of major importance to understand the impacts of climate variability and change on the population dynamics of emperor penguins and seabirds in general and to make robust predictions on the impact of climate change on marine predators.     

Field metabolic rates of black-browed albatrosses Thalassarche melanophrys during the incubation stage

Field metabolic rates (FMR) and activity patterns of black-browed albatrosses Thalassarche melanophrys were measured while at sea and on nest during the incubation stage at Kerguelen Island, southwestern Indian Ocean. Activity-specific metabolic rates of five albatrosses at sea (FMR_at-sea) were measured using doubly labeled water (DLW), and by equipping birds with wet-dry activity data loggers that determined when birds were in flight or on the water. The metabolic rates of four birds incubating their eggs (FMR_on-nest) were also measured using DLW. The mean±SD FMR_at-sea of albatrosses was 611±96 kJ kg⁻¹ d⁻¹ compared to FMR_on-nest of 196±52 kJ kg⁻¹ d⁻¹. While at sea, albatrosses spent 52.9±8.2% (N=3) of their time in flight and they landed on the water 41.2±13.9 times per day. The FMR of black-browed albatrosses appear to be intermediate to that of three other albatross species. Based on at-sea activity, the power requirement of flight was estimated to be 8.7 W kg⁻¹ (or 4.0×predicted BMR), which is high compared to other albatross species, but may be explained by the high activity levels of the birds when at sea. The FMR_at-sea of albatrosses, when scaled with body mass, are lower than other seabirds of similar body size, which probably reflects the economical nature of their soaring flight.     

Experimental increase of flying costs in a pelagic seabird: effects on foraging strategies,nutritional state and chick condition

A central point in life history theory is that parental investment in current reproduction should be balanced by the costs in terms of residual reproductive value. Long-lived seabirds are considered fixed investors, that is, parents fix a specific level of investment in their current reproduction independent to the breeding requirements. We tested this hypothesis analysing the consequences of an experimental increase in flying costs on the foraging ecology, body condition and chick condition in Cory’s shearwaters Calonectris diomedea. We treated 28 pairs by reducing the wing surface in one partner and compared them with 14 control pairs. We monitored mass changes and incubation shifts and tracked 19 foraging trips per group using geolocators. Furthermore, we took blood samples at laying, hatching and chick-rearing to analyse the nutritional condition, haematology, muscle damage and stable isotopes. Eighty-day-old chicks were measured, blood sampled and challenged with PHA immune assay. In addition, we analysed the effects of handicap on the adults at the subsequent breeding season. During incubation, handicapped birds showed a greater foraging effort than control birds, as indicated by greater foraging distances and longer periods of foraging, covering larger areas. Eighty-day-old chicks reared by treated pairs were smaller and lighter and showed a lower immunity than those reared by control pairs. However, oxygen demands, nutritional condition and stable isotopes did not differ between control and handicapped birds. Although handicapped birds had to increase their foraging effort, they maintained physical condition by reducing parental investment and transferred the experimentally increased costs to their partners and the chick. This result supports the fixed investment hypothesis and is consistent with life history theory.