Northern Prairie Wildlife Research Center
Nest, egg, and chick success calculations
Apparent nest success and Mayfield nest success are commonly used estimators of true nest success for piping plovers. In this section the two methods are compared, and Mayfield estimates are recommended if only one of the methods is to be used. Calculations of both estimators are illustrated by example in Appendix C.
A nesting attempt is considered successful if one or more eggs hatch. Apparent nest success is the proportion of observed nesting attempts that succeed.
Unfortunately, this simple and intuitive estimator overestimates true nest success whenever some nests fail before they can be discovered. The magnitude of this positive bias is related to nest search frequency and true nest success and thus has important implications for monitoring piping plovers.
Overestimates of success can inspire unjustified confidence in the status of a population and divert attention from effects of nest failure on population dynamics. Comparisons of true nest success among years, areas, or management strategies can be distorted when reporting apparent success. Finally, effects of nest search frequency and investigator skill, which vary among studies (and possibly among areas or years), confound comparisons of apparent nest success.
The Mayfield method was developed to correct for bias resulting from the destruction of nests before discovery (Mayfield 1975, Johnson 1979). Mayfield nest success is calculated from exposure-days and losses. One nest observed 1 day contributes 1 exposure-day; a loss is the destruction of a nest while under observation.
Because Mayfield estimates are based solely on events that befall a sample of nests after they have been discovered, the method performs well regardless of true nest success, and estimates are comparatively robust to variations in frequency or efficiency of nest searches (Johnson and Shaffer 1990).
However, despite advantages under most circumstances, Mayfield estimates do not always outperform apparent success estimates (Johnson and Shaffer 1990). For example, when nests that already have failed or hatched are located as easily as active nests, apparent estimates are equally valid and the Mayfield method, which considers only active nests, is comparatively inefficient.
This shortcoming, however, is not an issue when monitoring plovers on alkali lakes because it often is difficult to find nests that are no longer active, clearly ascribe them to breeding pairs, and find evidence of their fates (See E, Determining nest success in the field).
Both estimators of nest success require accurate determination of nest fates. Misclassification of nest fates as "successful" or "unsuccessful" introduces errors for obvious reasons. A more subtle bias results when fates of some nests are unknown.
Nests with unknown fates are left out of calculations for apparent nest success and are not treated as losses when Mayfield nest success is calculated. Thus, bias results unless fates of successful and unsuccessful nests are equally likely to be classified as "unknown," which is unlikely with piping plovers. The only way to obtain reliable estimates of nest success is to accurately determine the fate of nearly all nests studied.
Regardless of method, estimates of nest success should be accompanied by measures of uncertainty so readers can determine the precision - thus, the usefulness - of estimates. For this reason, calculation of standard errors and confidence intervals also are described and illustrated in Appendix C.
Calculating Mayfield chick success is similar to calculating Mayfield nest success (Appendix C), except that the sampling unit is either an individual chick or a brood. Regardless, the basis of such calculations should be communicated clearly. If a brood, the criterion for success is that ≥1 chick reach fledging age. Independence in fate among chicks of the same brood is assumed, although this may not be valid (Winterstein 1992) if the sampling unit is an individual chick. This assumption can be relaxed through a modified approach (Flint et al. 1995).
The hatching date counts as the first chick exposure-day (if the respective nest had been under observation). Otherwise, exposure-days begin with the first observation of chick(s).
Calculating Mayfield egg success poses an analogous situation.
Fledging rate calculation
Compared to nest success results, fledging rate (estimated mean number fledglings/pair) is a relatively unbiased measure of plover reproductive success if based on regular site visits and an accounting of breeding pairs as outlined in this protocol. An "unknown" category is seldom used or appropriate; young plovers either are accounted for during one to several visits when 18-20 days old, or they can be considered lost to decimating factors.
Causes of loss are difficult to determine, but timing of such attrition relative to chick age may be more readily assessed. Calculating pair success likewise is a straightforward matter. For examples see Appendix C.