Background/Question/Methods During spring time in the Sonoran Desert, columnar cacti like saguaro produce an abundance of nectar- and pollen-rich flowers, the seasonal timing of which is very predictable and mainly dependent on spring temperature. Flowering thus occurs in a wave, starting in Northern Mexico and moving north into Arizona. Since the geographic range of saguaro overlaps with the migration routes of many bird species we hypothesize that cactus nectar may be an important food source during spring migration. We took advantage of the fact that cactus nectar has an isotopic CAM signature (δ¹³C=–12.8 ‰ VPDB) very distinct from other (C₃) resources available at this time of the year (δ¹³C_C3=–24.9 ‰ VPDB). We collected samples of breath and blood from 34 bird species, migrants or local residents, captured in Southern Arizona in May and June. This time period encompassed the peak migration time as well as the local cactus bloom. Since carbon has very different turn-over rates in breath, plasma and red blood cells (RBC), we can determine the relative importance of cactus nectar as feeding resource from very recently (breath) to several weeks ago (RBC).

Results/Conclusions Breath samples varied in isotope values both within and among species. A mixing model that translates δ¹³C_breath into the relative contribution of CAM-derived resources to total assimilated nutrients reveals a reliance on CAM resources at the time of measurement ranging from 22.9 ± 15.6 % for the Black-headed Grosbeak to 75.0 ± 21.6 % for the Gila Woodpecker. Year-round residents generally had more positive δ¹³C_breath (-17.4 ± 0.52 ‰) than summer-residents (-19.1 ± 2.2 ‰) or migrants passing through the area (-19.4 ± 1.8 ‰) and thus appear to rely on CAM-resources to a higher extent. Saguaro utilization was lower in the month before, than at the moment of capture, as was reflected in the more negative δ¹³C_RBC compared to breath values. Migrants were relying mostly on C₃ resources in their wintering quarters, but also year-round residents and summer residents had been consuming only few CAM or C₄ resources. Our results show that columnar cacti provide an important food resource to the avian community in the American Southwest. Seasonal changes in its exploitation by avian consumers can be traced by stable isotopes with good temporal resolution.