CHANGES IN GUT STRUCTURE AND FUNCTION OF HOUSE WRENS (TROGLODYTES-AEDON) IN RESPONSE TO INCREASED ENERGY DEMANDS

We compared digestive tract structure and function in house wrens (Troglodytes aedon) feeding on crickets at two intake levels (5.18 g/d and 2.42 g/d). We increased the birds' intake through a combination of lowered air temperature and forced exercise. Apparent metabolizable energy coefficient (1 - (energy excreted/energy ingested)) was .771 in high-intake birds adapted to -9-degrees-C and 4 h exercise/d and .716 in low-intake unexercised birds at 24-degrees-C (P = 0.016). Small-intestine length was 21% greater in the high-intake birds than in low-intake birds (11.4 cm vs. 9.4 cm, P < 0.001). Stomach size, mouth-to-cloaca digesta retention time of aqueous digesta, and intestinal absorption of L-proline/cm did not vary between groups. A simple digestion model from chemical reactor theory is used to analyze these results and to deduce that an additional important adjustment was an increase in digesta retention time in the absorptive region of the gut, the small intestine. This would explain the observed increased digestive efficiency in the high-intake birds. The near-maximal rate of intake and digestion that we measured was among the highest ever recorded for a passerine. It exceeded by about 50% the field metabolic rate of parents during the reproductive season. Therefore, we reject the hypothesis that rate of digestion limits reproductive effort in house wrens.