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When evaluating avian reproduction, life history theory examines the trade‐offs between parental effort, the number and size of offspring, and the rate of nestling development. The growth rates and body sizes of developing birds vary geographically and can diverge with both latitude and migratory strategy. In terms of offspring size, growth rate can deviate in nestlings of the same or similar species due to the correlated influences of weather events, predation pressure, food availability, number of nestmates and parental provisioning. Furthermore, a longer photoperiod for species nesting at higher latitudes increases the duration over which a nestling can be fed each day, and increased nestling provisioning has been positively correlated with growth rate. Whether the amount of time a bird is fed during development drives this variation in growth rate and morphology is unknown. By removing supplemental environmental stressors (e.g. weather, predation) and standardizing feeding rate and environment, we explored the influence of daily duration of nestling provisioning on dark‐eyed junco Junco hyemalis nestlings. We hand‐reared 65 chicks of a sedentary junco subspecies J. h. carolinensis under both their natural photoperiod and the longer photoperiod of a closely related migratory subspecies J. h. hyemalis and compared growth rate, mass, morphology and the amount of food consumed. Average growth rate, fasted mass, wing length and total daily food consumption were all greater in birds hand‐reared under the longer, more northern photoperiod treatment. These findings suggest that increased daily photoperiod at higher latitudes may allow for greater total food provisioning and thus may play a role in the ability of parents in compressed breeding seasons to produce high quality offspring. This points to a trade‐off between provisioning effort and nestling growth rate in lower latitude (shorter photoperiod) populations and points to an important role of developmental plasticity on growth rate and morphology. 

Abstract Numerous factors influence the timing of spring migration in birds, yet the relative importance of intrinsic and extrinsic variables on migration initiation remains unclear. To test for interactions among weather, migration distance, parasitism, and physiology in determining spring departure date, we used the Dark‐eyed Junco (Junco hyemalis) as a model migratory species known to harbor diverse and common haemosporidian parasites. Prior to spring migration departure from their wintering grounds in Indiana, USA, we quantified the intrinsic variables of fat, body condition (i.e., mass ~ tarsus residuals), physiological stress (i.e., ratio of heterophils to lymphocytes), cellular immunity (i.e., leukocyte composition and total count), migration distance (i.e., distance to the breeding grounds) using stable isotopes of hydrogen from feathers, and haemosporidian parasite intensity. We then attached nanotags to determine the timing of spring migration departure date using the Motus Wildlife Tracking System. We used additive Cox proportional hazard mixed models to test how risk of spring migratory departure was predicted by the combined intrinsic measures, along with meteorological predictors on the evening of departure (i.e., average wind speed and direction, relative humidity, and temperature). Model comparisons found that the best predictor of spring departure date was average nightly wind direction and a principal component combining relative humidity and temperature. Juncos were more likely to depart for spring migration on nights with largely southwestern winds and on warmer and drier evenings (relative to cooler and more humid evenings). Our results indicate that weather conditions at take‐off are more critical to departure decisions than the measured physiological and parasitism variables.