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1999: Avian Dependence on Saguaro Cactus

The monumental saguaro cactus is the defining feature of the Sonoran Desert; it is also an important ecological resource in this unique ecosystem. The saguaro s floral nectar and fruit pulp contain nutrients and water resources and are available during the hottest and driest period of the summer. Thus, nectar and fruit pulp from saguaros may represent crucial water and energy resources for animals living in this desert Saguaros appear to be used to some extent as a source of food and/or water by almost every member of the Sonoran bird community and may be absolutely essential to some members, such as White-winged Doves. Migrant birds taking refuge in desert riparian areas may also benefit from the saguaro s water and nutrient resources. But remarkably, the importance of the saguaro for resident and migrant birds has never been quantified.

Saguaro populations are adversely affected by a number of human activities. Ecologists suspect that reductions in saguaro populations will impact bird populations. The questions are: by how much, and what species are likely to be affected? This summer, between May and September, we mist-netted birds in the Sand Tank Mountains of Arizona. During this period, we captured some 230 individuals of 37 species. About half of these species were Neotropical migrants and were using the desert riparian area as a stopover site. Upon capture, we obtained a breath sample and a very small blood sample from each bird. Birds were then released to continue their daily activities. From these samples we hope to obtain crucial information on the importance of the saguaro's nectar and fruit to the bird community.

We have identified natural tracers (carbon isotopes) in the fruit and floral nectar of the saguaro that allow us to track the movement of nutrients from the saguaro into the bird community simply by analyzing the carbon dioxide in the animals breath and blood. Our study will provide the first measurements of how extensively birds rely on saguaros for water and energy.

ABA support is essential to the success of our project. During the coming months, we will prepare and analyze the samples to see what information they provide about the foraging habits of birds in the Sonoran Desert. We also gratefully acknowledge the support of the National Fish and Wildlife Foundation and Swarovski Optik.

SPECIES CAPTURED

1. Gambel's Quail
2. White-winged Dove
3. Mourning Dove
4. Common Poorwill
5. Lesser NighthawK
6. Gilded Flicker
7. Gila Woodpecker
8. Ash-throated Fly catcher
9. Brown-crested Flycatcher
10. Western Flycatcher
11. Willow Flycatcher
12. Solitary Vireo
13. Warbling Vireo
14. Verdin
15. Cactus Wren
16. Rock Wren
17. Swainson's Thrush
18. Hermit Thrush
19. Northern Mockingbird
20. Curve-billed Thrasher
21. Wilson's Warbler
22. Yellow Warbler
23. Nashville Warbler
24. Townsend's Warbler
25. Black-throated Gray Warbler
26. Western Tanager
27. Green-tailed Towhee
28. Canyon Towhee
29. Lincoln's Sparrow
30. Black-throated Sparrow
31. Black-headed Grosbeak
32. Northern Cardinal
33. Pyrrhuloxia
34. Lazuli Bunting
35. Varied Bunting
36. Scott's Oriole
37. House Finch

Blair O. Wolf and Carlos Martinez del Rio
Department of Ecology and Evolutionary Biology
The University of Arizona in Tucson
September, 1999