Evaluation of a Reproductive Index to Estimate Grasshopper Sparrow and Eastern Meadowlark Reproductive Success

Abstract

We compared an index of reproductive success based on breeding behavior to actual nest fates of grasshopper sparrows ( Ammodramus savannarum ) and eastern meadowlarks ( Sturnella magna ) on 12 plots (4-ha). Concordance of results between the two methods was 58% for grasshopper sparrows and 42% for eastern meadowlarks on a plot-by-plot basis. The indirect method yielded higher estimates of reproductive activity than nest monitoring for the balance of the plots,. There was little evidence that brown-headed cowbird ( Molothrus ater ) parasitism influenced the estimates of reproductive success using the indirect method. We concluded that nests and about-to-fledge nestlings were missed during searches on some plots. It may be appropriate to use an indirect method to more efficiently survey territories and/or plots for species with hard-to-find nests or when monitoring large areas. Use of a reproductive index may be appropriate and more time-efficient than nest searching and monitoring for comparing management effects such as burning, grazing, haying, military training, and other localized disturbances that are likely to affect reproductive success of grasshopper sparrows and eastern meadowlarks. However, nest monitoring may be necessary for more precise estimates of productivity necessary for long-term monitoring. Nest monitoring results are also likely to allow for direct comparisons to results from other studies because the index method requires intimate knowledge of the species being evaluated - a factor that could lead to reduced precision because the experience level of technicians relying only on behavioral cues from study-to-study is likely to vary considerably.