Can diet composition estimates using stable isotope analysis of feathers predict growth and condition in nestling mountain bluebirds (Sialia currucoides)

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.