Nestling growth rate and food consumption increases under experimentally prolonged day length in a New World Sparrow

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 strategies. 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 in which a bird is fed during development drives this variation in growth rate and morphology is unknown. By removing supplemental environmental stressors (e.g., food availability, weather, predation pressure) 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.