Interpreting the results of nesting studies

Nesting studies are used to assess the production of birds and to evaluate nesting habitats. Most such studies involve finding nests in a given area and subsequently determining the proportion that hatched. Unfortunately, the results are often biased by unrecognized differences in the probabilities of finding successful and unsuccessful nests. The observed hatch rates of 1,900 nests of blue-winged teal (Anas discors) are presented to illustrate the relationship of hatch rates to time remaining until the nests should hatch. The Mayfield method of correcting for these biases is illustrated. Other examples demonstrate the possible effects of sampling procedures on observed hatch rates and nest density. J. WILDL. MANAGE. 42(3):471-476 Nesting studies are common in investigations of waterfowl and other birds. Most are undertaken to assess the production of breeding birds and to evaluate nesting habitats and the techniques of managing such habitats. The objectives are to determine hatch rates and density of nests in selected habitats. The procedures commonly used are searching selected areas to find nests and subsequently checking those nests to ascertain whether or not the eggs were hatched. Unfortunately, the nests of most species are initiated over periods of at least several days during which some of the nests may be destroyed. If nests are destroyed, many females will renest 1 or more times; consequently, it is common to find newly initiated nests after others of the same species have hatched. Continuous searches over such prolonged periods generally are impractical; hence, most studies involve periodic searches. If some nests were missed because they were initiated and destroyed between searches, the observed nest density clearly would be biased downward. Less obviously, but more importantly, the observed nesting success would be biased upward. We have restricted our discussion to these biases. These potential biases, among several commonly occurring in nesting studies, were recognized previously by Hammond and Forward (1956) and Mayfield (1960, 1961). Mayfield (1961) elaborated the method of estimating nesting success from nests observed during all or any portion of the period between initiation and hatch. The method has not been widely adopted. Mayfield therefore published the method again 14 yr later and noted correctly (Mayfield 1975:456) that "not every published report shows awareness of the problem. " At least 2 other investigators (Townsend 1966, Reed 1975) acknowledged the problem in waterfowl studies; Townsend used Mayfield's method in his analysis. Our purpose is to bring the potential biases associated with periodic searching to the attention of investigators who may consider undertaking nesting studies. Our studies will exemplify the magnitude of these biases. We will also illustrate how the inconsistency of the biases invalidates many comparisons commonly made within and among nesting studies. We will demonstrate Mayfield's method for obtaining better estimates of the success and density of nests. ' Present address: U.S. Fish and Wildlife Service, 1978 South Garrison Street, Denver, Colorado 80227. J. Wildl. Manage. 42(3):1978 471 This content downloaded from 157.55. 39.102 on Sat, 24 Dec 2016 05:19:18 UTC All use subject to http://about.jstor. org/terms 472 BIAS IN NESTING STUDIES Miller and Johnson We hereby acknowledge the support and guidance of H. K. Nelson, former Director, and W. R. Goforth, Director of the Northern Prairie Wildlife Research Center, in the studies which led to this paper. We are especially grateful for the use of the nest records provided by, and for the constructive comments of, H. F. Duebbert, K. F. Higgins, L. M. Kirsch, A. T. Klett, and J. T. Lokemoen. Our thanks to D. A. Davenport for assistance with computer programing, to C. W. Shaiffer for drafting the figures, and to H. F. Mayfield for reviewing an earlier draft of this report. Our special thanks to L. M. Cowardin for perceptive technical and editorial comments on the manuscript.