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 the use of saguaro (Carnegiea gigantea) nectar and fruit by migratory desert white-winged doves (Zenaida asiatica mearsnii). Saguaro resources had characteristically C-13-enriched CAM values (delta(13)C=-12.8+/-0.7 parts per thousand SD VPDB and -13.1+/-0.5 parts per thousand SD VPDB for nectar and fruit, 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 saguaro nectar and fruit was deuterium enriched (delta D=19.6+/-2.0 parts per thousand SD VSMOW and 48.4+/-1.6 parts per thousand SD VSMOW for nectar and fruit, respectively) relative to other water sources (ranging from -41 to -19 parts per thousand VSMOW). During the fruiting season, there was a positive correlation between delta(13)C in dove liver tissues and percent of saguaro in crop contents. A two-point mixing model indicated that during the peak 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 and delta D of body water showed a significant positive correlation, indicating 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 water losses by doves. Using dove carbon isotope data and a two end-point mixing 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 used to produce qualitative re-constructions of animal diets. Our study shows that they can be used to provide quantitative estimates of the now of nutrients from resources into consumers as well.