Insectivorous birds breeding in seasonal environments provision their dependent young during periods when prey diversity and abundance vary. Consequently, the composition and nutritional value of diets parents feed to their offspring may differ within and among broods, potentially affecting the condition of nestlings. In a population of mountain bluebirds (Sialia currucoides), we used two methods to estimate diet composition for individual nestlings: direct observation of provisioning using video recordings at 5 and 9 days post-hatch, and stable isotopes of the δ¹³C and δ¹⁵N in nestling feathers and prey followed by analysis with mixing models. We determined the macronutrient content (% fat and lean mass) and estimated the metabolized energy from each type of prey. We evaluated whether different methods of estimating diet composition would produce similar results, and if the types of prey nestlings ate at one or both ages affected their morphology, growth rates, or blood ketone concentration. We found that bluebirds fed their young 5 main types of prey: beetles, cicadas, grasshoppers, insect larvae, and spiders. Both observational and mixing model estimates of diet composition indicated that larvae are traded-off with grasshoppers, and that fewer larvae are provided to nestlings as the season progresses. In evaluating how diet influences individual growth and condition, estimates from direct observations had greater explanatory power than those from mixing models, indicating that diets rich in the most energy-dense prey (greatest fat content; cicadas and larvae) were associated with larger size and higher body condition, and faster rate of mass gain and growth of tarsus. Lower value prey had more limited, specific effects on nestlings, but may still be important dietary components. While isotopic methods produced estimates of diet composition that were generally informative, when applied to explain the growth and condition of nestlings they proved less useful. 

We studied mountain bluebirds breeding in nest boxes near Williams Lake, British Columbia, Canada (51°N, 122°W; 700 ‒ 1100 m a.s.l.; see O’Brien and Dawson, 2008 for further site details) in 2016. We measured the growth and condition of nestlings, and recorded provisioning by parents to determine what they ate on days 5 and 9 post-hatch. We collected feathers from nestlings and insects from the site to use for stable isotope analysis, to determine the isotopic ratios and concentration of C and N (δ¹³C, δ¹⁵N, [C], and [N]) in feather and prey samples. The isotopic signatures of feathers and prey were used in stable isotope mixing models to estimate the proportional contributions of different types of prey to the diets of nestlings.