By Jim Cox, Vertebrate Ecology Program Director
If you want to check the pulse of a Bachman’s Sparrow population to determine its health, two measurements are extremely important: (1) adult survivorship and (2) annual productivity. Taken together, these variables provide about 80% of the push behind annual growth in this species and collectively produce the best assessment of population health.
Vertebrate Ecology has developed robust methods for assessing adult survivorship over the past five years and has turned its attention the past two summers to looking at methods for measuring annual productivity. Simply counting the number of adults in an area may not provide a good snapshot because about half the adults may not be breeding. It’s also very difficult to find nests for this species.
Meanwhile, netting and banding young also is ineffective. In 2008, we spent 10 mornings monitoring 10 nets and captured only 3 juveniles. Based on the average number of juveniles netted per net hour, about 75 total days of operation would be needed to net just 30 juveniles.
Our latest attempts focuses on assessing productivity by looking for recently fledged young and also monitoring behavioral cues among adults. We have established 16, 2.5-acre plots on the Wade Tract and slowly walk each plot weekly in a systematic manner. Within each plot, we look for paired adults, adults carrying food and nesting material, and bob-tailed young that are only 4-5 days out of the nest. Adults also give a distinctive alarm note when biologists get close to a nest with young, and this too can be used as an indication of nesting activity.
By tallying these observations throughout the summer months, an index to productivity can be calculated and used to rank each plot from low to high. On the Wade Tract, the west side has had lower productivity in both years of study (see accompanying figure). We are quantifying vegetation characteristics associated with each plot in hopes of finding correlations between vegetation and productivity. As important, our index to productivity also seems to have a connection with adult behavior as well. Color banded males on the west side of the Wade Tract show less site fidelity than males on the east side of the tract, a result that makes some sense if you’re a male trying to pass along your genes. The west side may have lower habitat quality, so you would want to move into a higher quality setting if you had a chance.
In July, the VE program also made two presentations at the American Ornithologists’ Union Meeting in Jacksonville and led a pine-grassland field trip for those attending the conference. Professional birders from England, Oregon, and other distant lands had a chance to see field procedures used to monitor Bachman’s Sparrows, Red-cockaded Woodpeckers, and Brown-headed Nuthatches.