Use of saguaro fruit by white-winged doves: isotopic evidence of a tight ecological association

We report the use of stable isotope and crop content analyses to quantify t he use of saguaro (Carnegiea gigantea) nectar and fruit by migratory desert white-winged doves (Zenaida asiatica mearsnii). Saguaro resources had char acteristically C-13-enriched CAM values (delta(13)C=-12.8+/-0.7 parts per t housand SD VPDB and -13.1+/-0.5 parts per thousand SD VPDB for nectar and f ruit, respectively) relative to other food plants used by doves (delta(13)C (C3)=-24.9+/-3.3 parts per thousand SD VPDB). The water contained in saguar o nectar and fruit was deuterium enriched (delta D=19.6+/-2.0 parts per tho usand SD VSMOW and 48.4+/-1.6 parts per thousand SD VSMOW for nectar and fr uit, respectively) relative to other water sources (ranging from -41 to -19 parts per thousand VSMOW). During the fruiting season, there was a positiv e correlation between delta(13)C in dove liver tissues and percent of sagua ro in crop contents. A two-point mixing model indicated that during the pea k of saguaro fruit use, most of the carbon incorporated in dove tissues was from saguaro. Desert white-winged doves appear to be saguaro specialists. Averaged over the period when doves were resident, saguaro comprised about 60% of the total carbon incorporated into dove tissues. Tissue delta(13)C a nd delta D of body water showed a significant positive correlation, indicat ing that doves were using saguaro as a source of both nutrients and water. However, at the peak of saguaro utilization, the doves' body-water delta D was more positive (by about 20 parts per thousand) than saguaro fruit water . We hypothesize that this enrichment is due to fractionated evaporative wa ter losses by doves. Using dove carbon isotope data and a two end-point mix ing model we estimate that, on average, doves consume the equivalent of 128 saguaro fruits per season; each fruit contains on average 26.0+/-14.8 g SD of pulp (wet mass) of which 19.4 g is water. Stable isotopes have been use d to produce qualitative re-constructions of animal diets. Our study shows that they can be used to provide quantitative estimates of the now of nutri ents from resources into consumers as well.