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-Mayfield's method for calculating the success of a group of nests is examined in detail. The standard error of his estimator is developed. Mayfield's assumption that destroyed nests are at risk until the midpoint of the interval between visits leads to bias if nests are visited infrequently. A remedy is suggested, the Mayfield-40% method. I also present a competing model, which recognizes that the actual destruction date of a failed nest is unknown. Estimated daily mortality rates and standard errors are developed under this model. A comparison of the original Mayfield method, the Mayfield-40% method, and the new method, which incorporates an unknown date of destruction, shows that the original or modified Mayfield method performs nearly as well as the more appropriate method and requires far easier calculations. A technique for statistically comparing daily mortality rates is offered; the one proposed by Dow (1978) is claimed to be misleading. Finally, I give a method for detecting heterogeneity among nests and an improved estimator, if it is found. Received 5 March 1979, accepted 28 July 1979. THE well-being of an avian population lies in the delicate balance between natality and mortality. Biologists attempt to infer the status of a species by estimating rates of births and deaths and, through their comparison, determining if the former are sufficient to offset the latter. For most populations of wild birds, none of the crucial characteristics of population dynamics is easy to measure. One component of natality that seems easy to gauge is the percentage of nests that hatch, which is often used as an indirect measure of reproduction. Mayfield (1961) has demonstrated, however, serious error in the ordinary method of determining this rate: dividing the number of nests under observation into the number of those that ultimately hatch. To overcome the difficulties he recognized, Mayfield (1961, 1975) developed an alternative method for calculating hatch rate. In it he accounts for the fact that normally not all nests are under observation from the day of. initiation but are discovered at various stages of development. Nests found in a late stage are more likely to hatch than those found in an early one, because they have already survived part of the requisite time. Combining all nests, regardless of stage of development, and calculating an apparent hatch rate will result in a severely biased estimator. Mayfield's method places all nests on a comparable basis by using only information from the period during which a nest was under observation. The length of that period he termed the exposure, although risk may be a more appropriate term. Thus, a nest that was found on 10 May and was still active on 18 May had survived 8 days of exposure. Had it been destroyed by 18 May, Mayfield would credit the nest with 4 days of exposure, under the assumption that it was at risk for half the period. 651 The Auk 96: 651-661. October 1979 652 DOUGLAS H. JOHNSON [Auk, Vol. 96 From a group of nests, Mayfield calculates the total exposure in nest-days. This number is divided into the number of nests that were destroyed while under observation. The resultant value, expressed as losses per nest-day, is the estimated daily mortality rate of nests. For example, in Mayfield's (1961: 258) analysis of Kirtland's Warbler (Dendroica kirtlandii), 154 nests seen during incubation represented a total exposure of 882.5 nest-days. (Mayfield's data have been reanalyzed here; some results differ slightly from his original presentation.) Thirty-five nests were lost (destroyed or deserted), yielding a daily mortality rate of 35/882.5 = 0.04 losses per nest-day. To determine the probability that a nest survives the entire period of incubation, one must know the length of that period; for the Kirtland's Warbler it is 14 days. The probability of survival for one day is 0.96 (=1 0.04), so the probability of surviving throughout the 14-day incubation period is 0.96 times itself 14 times, or 0.9614 = 0.56. Although the Mayfield method is a major advance in treating nesting data, it has been criticized (Green 1977) because of its assumption that the population is homogeneous, i.e. all nests are subject to the same rate of mortality. In addition, Mayfield provided neither variance estimates for his mortality rate nor tests of the underlying assumptions. The present paper is intended to augment Mayfield (1961 and 1975). In it I derive his estimator, which he developed heuristically, in a more formal context. A standard error for his estimator can be calculated from this derivation. The implications of Mayfield's assumption that nests are at risk until midway between visits are considered in detail. I also propose a more realistic model, which does not require the midpoint assumption. Estimators of the daily mortality rate and its standard error are obtained under this model and compared to those of Mayfield. Finally, I discuss the importance of variation in daily mortality rates, from both identifiable and nonidentifiable causes. Methods of detecting such variability and treating it, if it exists, are presented.