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. Para juveniles altriciales, salir del nido es un paso abrupto hacia un ambiente desconocido. A pesar del incremento en los estudios investigando el periodo posterior a la salida, nuestro conocimiento actual de las causas y consecuencias de la supervivencia luego de salir del nido es fragmentada. Aquí, revisamos la literatura sobre la supervivencia de aves juveniles altriciales posterior a su salida del nido, con el fin de responder las siguientes preguntas principales: es la baja supervivencia luego de salir del nido un cuello de botella en el ciclo reproductivo altricial? Que sabemos sobre los factores causales últimos y proximales, como las relaciones tróficas (comida y depredación), condiciones del hábitat o factores abióticos que influencian el periodo posterior a la salida del nido? Analizamos estimados semanales de supervivencia de 123 series de datos basados en estudios de 65 especies, cubriendo 1–13 semanas posteriores a la salida del nido. Como patrón general, la supervivencia de los volantones fue baja durante la primera semana posterior a la salida (mediana de la tasa = 0.83) y mejoro rápidamente con el tiempo luego de la salida (semana 4 mediana mediana de la tasa = 0.96). Para especies que anidan en el suelo, la supervivencia inmediatamente después de salir del nido fue similar a la supervivencia de huevo a volantón. Para especies que anidan por encima del suelo, supervivencia durante la primera semana posterior a la salida fue substancialmente menor que durante los periodos de anidación y periodos tardíos posteriores a la salida. Consecuentemente, el periodo temprano posterior a la salida del nido es un cuello de botella con una marcada mortalidad elevada para la mayoría de especies altriciales. Depredación fue la principal causa proximal de mortalidad. El hábitat, variación anual y estacional del ambiente y la condición de los volantones han sido reportados como factores que afectan la supervivencia posterior a la salida del nido. La supervivencia individual dependió fuertemente de caracteres físicos como masa, longitud del ala, los cuales probablemente influencian la habilidad de los volantones de escapar a los depredadores. Relaciones tróficas en varios niveles son el factor ultimo de adaptación de caracteres relevantes para la supervivencia durante los periodos previos y posteriores a la salida del nido. La dinámica espacio-temporal de los recursos alimenticios determinan el desarrollo físico de los juveniles y por lo tanto su rendimiento después de salir del nido. Sin embargo, los depredadores pueden causar selección rápida y eficiente para los caracteres de los volantones y de las decisiones de reproducción en los adultos. La intensidad y la duración de la inversión parental posterior a la salida del nido también influencia la supervivencia de los volantones. La mortalidad posterior a la salida del nido es por lo tanto no aleatoria y una perdida inevitable. Los caracteres y las estrategias relacionadas con la salida del nido y la etapa posterior a esta salida, crean diferencias grandes en la aptitud y por lo tanto son una parte integral pero poco comprendida de la estrategia de reproducción altricial. Many physiological and behavioral adaptations within the gradient from precocial to altricial avian reproduction have been described (Ricklefs 1968, Ricklefs and Bloom 1977). Precocial species produce well-developed hatchlings able to move and forage on their own. Parents provide limited post-hatching care, e.g., guiding young to food resources and protecting them from predators (e.g., Dreitz 2009, Rickenbach et al. 2011). The period of chick growth and coherence of families may nevertheless be remarkably long and the precocial reproductive strategy is generally described as “slow”, with relatively slow chick growth rates and low parental investment per chick. Among altricial species, reproductive investment appears to be more equally balanced between the sexes. Mates can cooperate in provisioning broods and, as a result of intensive parental care, nestlings grow at rapid rates (Starck and Ricklefs 1998). One potential fitness advantage of the “fast” altricial breeding strategy is more than can be per A large of and behavioral traits is related to the altricial reproductive strategy the altricial fledging is a species, the timing of fledging with the age and the physical development of variation in traits is et al. the stage at which juveniles nests is and in to The duration and of parental after young fledge also species. the of the and of reproductive altricial have been with a on the main have been the studies within and species may selection on from of and clutch to the timing of breeding and food provisioning (e.g., and on and been found to selection on reproductive e.g., growth rates and timing of fledging avian is based on habitat and the of resources and and et al. and the of the and adaptation of nestling traits in to and habitat been (Ricklefs et al. et al. the of the altricial and the of fledging an abrupt and fledglings juveniles unknown and with the resources and in the abiotic environment, such as and to may may result in mortality during the between and was the first to the post-fledging period of altricial as a stage with for reproductive and relevant in dynamics and and and the of the early post-fledging stage in the survival of altricial species are known to provide parental after juvenile parental and causal is studies have on the period from to fledging or the period from fledging to first As the consequences and of post-fledging survival are unknown for most species. the period of post-fledging have been the of following juvenile and their after fledging. a of the altricial been the have for the of young after leaving to reproductive large numbers of to survival rates and with (e.g., and and we more the of juvenile altricial after the duration of the post-fledging period time between fledging and survival during period, and the of parental to have of juvenile survival during the post-fledging on et al. or based on estimates of survival and post-fledging mortality is during the first weeks after leaving survival is low to of in species and Thus, mortality during the post-fledging period likely such as growth rates and age Here, we review the literature post-fledging survival of the young of altricial species of to survival and the causes of mortality after leaving the general related to age and the of What is known factors such as habitat conditions, trophic or abiotic factors acting in the early post-fledging period? the general traits are to adaptation et al. we juvenile and parental strategies may post-fledging survival as an integral of reproductive and We the of the post-fledging with to the of in the of young and the reproductive of our was to provide a of the of the post-fledging period as a stage in the of the altricial reproductive strategy. We our review on studies juvenile survival rates with on variation in survival to time post-fledging. We and are related to the step of fledging. 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For most species of birds, young fledge have physical and and are fledge at of adult mass and in species nests at developmental and have growth of data from and variation species with For ground-nesting species and species with nestlings nests at of adult the for species in nests and and the developmental stage at which young fledge is based on Predation is the factor However, the survival of fledglings to The of leaving nests at an early developmental stage may be a in of survival for the studies are to for both the nestling and fledgling et al. and to rates of development and of to various physical from the literature are not in of time trophic and species are and estimates of survival are on the first weeks post-fledging For week 4 post-fledging and data are and from species. of selection of fledglings mortality during the fledglings poorly to the of the is and analyzed species of and found fledging is a the between and fledgling developmental stage is Thus, young of species with rates of nests at developmental and are their growth rates are than of species with lower rates of predation. and and an between the of the nestling period physical development is and the of of nests to and fledging mass breeding on the or in nests fledge at an developmental stage of adult than species breeding in nests or and of adult species have rates of than or species. to studies developmental condition of fledglings is of for post-fledging survival. adult mass and wing are at predation. effects are substantial an of the nestling period result in markedly improved post-fledging survival and reproductive Thus, for an individual an developmental stage for survival. 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However, the in survival rates strongly species and within species data the first week variation in survival rates was large = the low of the data weekly survival rates between and for species and survival rates can be low The most main in the is the time in survival rates for the first week post-fledging lower than for weeks and We found a substantial of the trophic of species. species lower survival than species = The development of weekly survival rates also species with species and species with nests lower survival than species with nests and species. The not to the mass of adult mass no on survival studies survival was for mortality during the first after leaving nests et al. and 2009, et al. et al. a in survival can as a of fledging. Various potential the general have been most the effects of habitat, and is et al. 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For the rates and and As in parental breeding have been the of studies of the and reproductive performance of species (e.g., et al. and and have survival to fledgling as fledging e.g., et al. et al. et al. have analyzed the potential (e.g., timing of breeding or post-fledging parental for a species, the of variation in parental reproductive their with and the reproductive is the first of the of parental breeding the of the duration of parental during the post-fledging period of was a and the duration of post-fledging parental markedly and was the of post-fledging survival and the of Parents for their fledglings for more than of post-fledging a period of parental care, the in chick mortality was lower and timing effects on and found a in survival of juveniles with during the post-fledging Thus, have to the of for first broods the of the of The of in the the and on the first to the of juvenile survival. and studies of the adaptation of clutch nestling and timing of breeding have into the reproductive of the species. the early was with of the causal the the the most and and studies of individual growth performance was not the most from the seasonal development of resources to have the fitness et al. and of the breeding and nestling of a the the of predation. than the of nestling may of juvenile birds, a fledgling in their and However, the effects of food and a families of during the first weeks after fledging families and of found the first after leaving nests a bottleneck in the of was later with mortality rates during the first after leaving nests et al. Predation is the of fledgling in selection for fledging mass and early differentials for fledging mass are later in the rates are et al. 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Parental post-fledging parental care, and adaptations related to timing of breeding and the age and stage at which fledge likely have substantial effects on the survival of young after fledging. Post-fledging mortality is therefore not random and inevitable. The traits and strategies related to fledging and the post-fledging stage create large variation in survival and, therefore, fitness and are integral parts of the reproductive strategy of altricial species. trophic both food and are proximate of post-fledging survival and adaptation of reproductive The to nestlings post-fledging survival nestling growth and fledgling and to the of food and the provisioning performance of However, of fledglings is a proximate cause of mortality. the effects of variation in food resources and a in the reproductive traits of altricial species of in general and of post-fledging survival in The reproductive of is to both the reproduction and of species and to the of predators reproductive are to with of similar species post-fledging mortality is substantial influence rates such as and annual survival et al. Post-fledging mortality is likely to the mortality during in the first (e.g., and 2009, a of the of species, is a between and fledging age and development (e.g., and with the ability of fledglings to escape predators individual post-fledging survival strongly with development and wing acting on the duration of nestling and post-fledging survival of of fledglings such as mass or wing and development be for the effects of variation in nestling development on post-fledging behavioral performance and survival. in at and of individual post-fledging survival poorly and review can provide a to Individual and behavioral traits post-fledging survival are species may provide an to the of traits and However, our review the data for species are and adaptation of parental reproductive traits reproductive have post-fledging survival in to variation in parental for relationships be and most fledglings survival during the first after leaving However, the general with species For the in survival rates after fledging is than for of adult to survival during the nestling period, post-fledging survival of ground-nesting species of not to for species or in survival during the first week post-fledging been found to be markedly lower than during the nestling period or the later post-fledging period, a However, post-fledging survival is strongly individual and and the breeding of of have the reproductive and of food resources as as of predators can create quick and selection et al. parental breeding and individual result in fitness with the of reproductive traits and and and strongly a rapid in our review and and the developmental of nestlings and wing result in post-fledging survival. between and species with be to the for the of the development of physical traits is in the data are post-fledging and for are juvenile for parental in of of and the post-fledging period, both the to for parental and the of predators may be young are no to a are in our knowledge of the and of families of during the post-fledging seasonal young fledge at a relatively early stage may be can move to with resources (e.g., and may the of young to predators and of the post-fledging period are limited and following juvenile for increasing with large survival rates with large is a of from to the with the duration of the are the increasing of estimates for later survival estimates large and for is and However, at the and levels rapidly with the of to be studies to have been general as a to and We studies in (e.g., or to (e.g., age of our knowledge of the pre- and post-fledging stages. of post-fledging survival in a (e.g., and et al. 2011). such as food et al. et al. have variation in the to the and consequences for post-fledging survival. A and variation in the duration of post-fledging parental and into the of post-fledging parental The for selection for fledging at developmental the most the the proximate the consequences of the survival of altricial young are the nestling and post-fledging to and trophic improved for the to and to general of breeding altricial are et al. Many of the causal the adaptations may during the post-fledging period, and the of juveniles from fledging to can reproductive et al. et al. and and 2011). the of reproductive traits may be the of fledglings is as a for the post-fledging of reproductive performance is also for of dynamics (e.g., et al. and studies are we be able to the post-fledging period into a covering from to into the breeding For on the and or on of the we and and data on egg-to-fledging survival of various species. We are to and for to the data in and and for the The are no of