Photo © Al Tuttle

What method do you use to estimate nest success?

When participants use the grid-based “Multiple Visits” method of data entry, scientists can estimate daily nest survival.

Accurately estimating nest success (the fraction of observed nests that fledge at least one young) is a critical goal of any nest monitoring program. True nest success is almost never the proportion of nests that fledged offspring, particularly for birds that do not nest in nest boxes. This is because nests are not all found at the same stage of nesting. You can imagine that a nest that is destroyed before the entire clutch is laid has a very low chance of being found, whereas a nest that survives until fledging is likely to be noticed, both because it is around longer and because the parents become more conspicuous as they begin feeding and defending their brood. For this reason, nests that survive the longest are most likely to be found and nests that fail very early are often missed entirely! Since this detection bias means that we find more successful than unsuccessful nests, it becomes important to correct for the fact that our data will tend to overestimate nest success in a population.

To eliminate this bias the new data entry system uses widely accepted methods to achieve accurate estimates of nest success. The simplest of these was developed by Mayfield (1961, 1975, see calculation below). All of these methods are based on daily nest survival (i.e., the proportion of nests that did not fail on a given day, while the nests were under observation). The total number of nest-days of observation is called exposure days. In order for researchers to use the Mayfield Method of analysis to estimate nest survival, we need to have a chronological record of each visit to the nest, which can be reported using the “Multiple Visits” method.
These more sophisticated analyses will also allow researchers to examine how factors such as habitat, seasonal effects, or experimental treatments affect the probability of nest success.
Using Mayfield’s method, nest success is calculated as:

Mayfield Estimate of daily nest survival = 1 [(number of nest losses)/(total exposure days)]
Estimate of nest success = (daily nest survival)n

where n = length of the nesting period (just incubation for precocial species, and incubation and nestling period for altricial species)