The Trade‐Offs Between Digestion Rate and Efficiency in Warblers and Their Ecological Implications

Abstract

Frugivory in birds is associated with rapid gut passage whereas insectivory is associated with slower gut passage. This is interpreted by some avian ecologists to reflect an inherent digestive constraint on diet selection, but it could also result from dietary acclimation. We predicted that Yellow—rumped Warblers (Dendroica cornata) acclimated to fruit—, insect—, and seed—based diets would exhibit retention time that increase in that rank order, because this is the rank order of retention time across species that eat these types of food. We also predicted a trade—off between rate of processing (the inverse of retention time) and extraction efficiency. This is based on the assumption that digestive enzymes or absorptive capacity occur in the gastrointestinal tract at levels that are not in great excess, and so less contact time between enzymes and digesta reduces the extraction efficiency. To test these predictions, we measured retention time (using aqueous and lipid—phase inert markers) and extraction efficiency of glucose, sucrose, leucine, starch, and the lipid glycerol trioleate (using radio—labeled nutrients and inert markers). Our results were in accordance with predictions. Mouth—to—anus total mean retention time (TMRT) of Yellow—rumped Warblers acclimated to fruit—, insect—, and seed—based diets were, respectively, 46 + 9, 62 + 6, and 114 + 9 min for polyethylene glycol (PEG, an aqueous marker): results were similar for the lipid phase marker in most cases. But Yellow—rumped Warblers that were switched suddenly to an alternate diet did not readjust TMRT when tested 2 h later or did so incompletely. We found no diet—related morphological changes in the digestive tract, and thus attribute these results to changes in gut motility. Extraction efficiency was uniformly high across all diets for glucose (88 + 1%), but varied among diet groups according to out prediction for leucine (range 82—94%), sucrose (58—85%), starch (9—48%), and lipid (18—82%). We review how features of the Yellow—rumped Warbler's system permit relatively high energy uptake across a wide variety of food types. The most notable constraining feature was a low starch hydrolysis rate, probably restricting them from relying on starchy foods. Thus, digestive strategy (i.e., a combination of retention time of food in the gut and digestive efficiency) somewhat determines diet, but in more respects diet determines strategy.

Seasonal variation in body mass and energy budget in Chinese bulbuls (pycnonotus sinensis)

“…For birds maintaining constant body mass and body composition, time-averaged energy intake equals time-averaged energy use (Hammond and Diamond, 1997). This balance depends on the interplay between the intake and digestive processing of matter and energy and their allocation among diverse functions, including thermoregulation, growth and reproduction (Afik and Karasov, 1995;Starck and Rahmaan, 2003;Caviedes-Vidal et al, 2007). Increased food intake, hypertrophy of the gastrointestinal tract and the consequent increased absorption of nutrients may be viewed as responses by wintering birds to physical and biotic seasonal habitats (Novoa et al, 1996, McWilliams andKarasov, 2001).…”

A growth cost of begging in captive canary chicks

“…To assess whether a growth cost might reasonably be equated with the fitness cost described theoretically, I also examined the correlation between nestling growth and survival to independence. Finally, I investigated whether increased begging potentially incurred digestive costs by measuring the rate of fecal sac production, a known correlate of digestive efficiency (30).…”

Effects of nicotine on the digestive performance of nectar-feeding birds reflect their relative tolerance to this alkaloid

“…These results are in line with the nicotine tolerance of white-eyes, which was comparable to that of sunbirds at low sucrose concentrations (Lerch-Henning and Nicolson 2013). In addition, since assimilation efficiency is linked to retention time (Afik and Karasov 1995), it is not surprising that we found that high nicotine affected negatively both parameters. However, Cape white-eyes that are legitimate pollinators for Aloe vryheidensis are tolerant to phenolics in this nectar (Johnson et al 2006).…”