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Tucson Audubon Society
Monitoring Birds at the North Simpson Restoration Site
by Kendall
Kroesen and Scott
Wilbor
Since February of 2001, staff and volunteers have done quarterly bird surveys along the lower Santa Cruz River at Tucson Audubon’s North Simpson habitat restoration site. This article gives some preliminary results of those surveys.
The 1,700-acre site where restoration is occurring is northwest of Marana, just south of the Pinal Air Park and the county line. The property is owned by the City of Tucson and managed by Tucson Water. Tucson Audubon has a 99-year right-of-entry agreement with the City for the purpose of doing habitat stabilization and restoration work. The restoration in the part of the site along the southwest side of the river has been paid for by two grants from the Arizona Water Protection Fund (AWPF). Bird monitoring and other measures of success are part of the AWPF funding requirements.
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The objective of the bird monitoring was to document changes in bird numbers and diversity. Spring and summer surveys along the 1.4-mile survey area consist of eight point counts, an effective monitoring approach during nesting season when most birds are defending territories. On fall and winter surveys, the counts are done in a series of transects since birds may forage more broadly on the landscape in these seasons. While results based on the first four years are not statistically significant, they do indicate some trends.
Since 2001, summer bird diversity increased from 37 to 51 species (Figure 1). The increase may not yet be related to planting and seeding efforts, as most new plants are still relatively small. Some of the increase may have to do with the natural growth of the willow and cottonwood canopy in the absence of cattle, which were fenced out by the end of 2001. Winter flooding has scoured sandy benches along the river and hastened the germination of new cottonwoods and willows. Yellow-breasted Chat, Bell’s Vireo, and Song Sparrow have shown notable positive numerical trends during the study period. Bullock’s Oriole and Yellow Warbler, both canopy associated species, were more variable but indicate a possible positive trend. Brown-crested Flycatcher and Hooded Oriole were first recorded in 2004.
Bird diversity in the winter has also increased. Figure 2 shows an increase from 32 to 45 species detected during winter surveys. This may have occurred for reasons similar to the summer increases.
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Spring and fall results are more variable, showing no clear increase or decrease in species diversity. This may be due to the presence of migrating birds during these seasons. The number of migrating birds in the project area varies from day to day. Some migrating species are present every year but in relatively low numbers. Those species may, by chance, be detected in some years and not in others.
We predicted an increase in winter sparrow and towhee diversity due to their use of annual forbs and grasses which responded quickly to removal of cattle. Sparrow/towhee diversity increased from 5 to 9 species during the second winter (2002) and remained at 9 in the winters of 2003, 2004, and 2005. The species most often seen were White-crowned, Song, Lincoln’s, Brewer’s, Rufous-crowned and Chipping Sparrow, and Abert’s and Green-tailed Towhee. Numbers of individuals of many of these species varied quite a bit over the five winters, suggesting that these numbers may be affected by population mechanisms on northern breeding grounds, or the tendency of winter flocks to range widely over the landscape.
Another winter species, the Ruby-crowned Kinglet, has increased steadily over five winters from 7 in 2001 to 26 in 2005 (maximum number seen on a given winter survey). This, again, may be due to increased tree canopy due to cattle exclusion, and to good cottonwood-willow recruitment.
The presence of Yellow-billed Cuckoo in the summers of 2004 and 2005 may be a significant indication of habitat recovery at the site. Before 2004 it was detected only once before, likely a migrating bird. But in summer surveys in 2004 and 2005, several individuals were detected. While nesting status has not been firmly determined, the sightings persisted through July in both years and into at least early August this year, suggesting that they are in fact nesting. Cuckoos are a mid-story species requiring large habitat patches. As near-river restoration plantings mature, riparian vegetation width should increase, thus providing more suitable cuckoo habitat.
A concern raised by the bird surveys is the large number of Brown-headed Cowbirds observed during the spring and summer surveys. A large dairy near the site provides food for these birds, and the restoration site provides an area where they may parasitize nests of other birds. There is the possibility that even though species diversity is increasing at the site, reproductive success for parasitized species may be low. We are working to find ways to research not only the avian diversity and numbers at the site but their reproductive productivity as well.
Overall, we see the restoration project helping to restore bird populations that were lost or greatly reduced during the last century along the lower Santa Cruz River. The positive trends and new occurrences of avian species listed as Priority Species by the Sonoran Desert Conservation Plan (Bell’s Vireo, Abert’s Towhee, Yellow-billed Cuckoo and Swainson’s Hawk) further indicate the importance of this project.
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This page was updated on 02/22/06