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.

During northward migration, blackcaps that arrive to refuel at stopover sites in Israel's Negev Desert have reduced masses of organs that are important in food digestion and assimilation. We tested several predictions from the general hypothesis that smaller organs of digestion (small intestine and pancreas) and nutrient assimilation (liver) bring about a lower capacity to consume food and that the organs must be restored before blackcaps can feed and digest at a high rate. We used a fasting protocol to create a group of blackcaps with reduced intestine and liver mass (reduced by 45% and 36%, respectively) compared with controls fed ad lib. Because most of the small intestine's biochemical digestive capacity reside in enterocytes found on villi, we predicted and found that reduced intestinal mass in fasted blackcaps related mainly to changes in enterocytes rather than other cells and tissues such as nonabsorptive crypt cells or underlying muscle. Because migrating blackcaps that stop over to feed begin to increase in body mass only 2 d after arrival, we predicted and found a similar recovery period in blackcaps that were first fasted but then refed--the organ mass, structure, function, and ability to consume food was restored after 2 d of feeding. Another group of food-restricted blackcaps (fed at one-third ad lib. level) lost similar amounts of body mass as fasted blackcaps but had much greater capacity to consume food than fasted blackcaps, and so we predicted that they would exhibit little or no reduction in alimentary organs relative to controls fed ad lib. A surprising result was that, as in fasted blackcaps, in food-restricted blackcaps, the decreases in masses of small intestine, liver, and pancreas were proportionally greater than the decreases in body mass or in masses of nonalimentary organs (heart, pectoralis). Food restriction, like fasting, caused a decrease in amount of intestinal mucosa and an alteration in the phenotype of enterocytes. These results are thus not consistent with the general hypothesis, and although they can be rationalized by assuming that blackcaps fed ad lib. have excess digestive capacity, it may also be that the physiological process or processes limiting very high feeding rate lie elsewhere than in the digestive system.

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Seasonal temperate zone breeders respond to increasing day length to anticipate the approach of spring breeding conditions. Other (supplementary) environmental cues, such as temperature and precipitation, were historically thought to play unimportant roles in reproductive timing. We demonstrate variation in reproductive timing across small geographic distances by examining the vernal testicular recrudescence of adult song sparrows (Melospiza melodia morphna) breeding in coastal (0-10 m elevation) and montane (280-1220 m elevation) habitats. Each year, these birds experienced the same photoperiod, but were exposed to different supplementary cues that varied with altitude. Coastal birds experienced warmer and more stable temperatures during late winter and early spring than did montane birds. We measured bud opening, emergence of new green shoots, and arthropod biomass to monitor the pace of spring's approach. New spring shoots emerged 2 months earlier on the coast than in the mountains and buds on flowering trees and shrubs also tended to open earlier at the coast. Arthropod biomass was similar in both the mountains and the coast during early spring, and began to increase in early summer. Reproductive morphology (i.e. testis volume and cloacal protuberance length) developed up to 2 months earlier on the coast than in the mountains. Testicular recrudescence occurred earlier on the coast in most years and proceeded at a faster rate in 1 year. Circulating levels of luteinizing hormone, follicle stimulating hormone and prolactin increased through the season, but did not correlate with differences between sites. Both populations responded similarly when exposed to identical photoperiodic cues in the laboratory. Therefore, we suggest that an integrated response to cues characteristic of location and elevation account for differences in patterns measured in the field.