Birds unadapted to prolonged fasting may not follow the mass loss pattern characteristic of fasting‐adapted species

Abstract

Birds, such as penguins and long distance migrants, which are considered adapted to long fasts, sequentially oxidize endogenous carbohydrates, lipids and proteins to meet energy demands. The pattern of nutrient use, metabolic biochemistry, and body mass changes during fasting are generally characterized in three phases; I ‐ post‐absorptive phase; II ‐ protein sparing phase; and III ‐ lethal phase. Little is known about how birds that are not so adapted use endogenous fuels during fasting or food restriction. We hypothesized that those small birds, that do not regularly undergo long fasts have truncated, but otherwise similar fasting phases and sequential changes in fuel oxidation and blood metabolites to fasting‐adapted species. We tested our hypothesis in house sparrows (Passer domesticus) using artificially enriched 13C stable isotope tracers (glucose, palmitic acid, and glycine) and measuring 13CO2 in their breath. We predicted that fasted birds, dosed with 13C enriched tracers, have 13CO2 enrichments in exhaled breath reflecting the sequential changes in fuel oxidation during fasting, each phase being represented by a specific tracer. Our initial data show continuous, parallel oxidation of the tracers during fasting, with no clear division into phases, rather than sequential changes of fuel oxidation similar to fasting‐adapted species and, therefore, do not support our hypotheses.