How does NestWatch estimate nest success?

Nest success can be very simply defined as the fraction of observed nests that fledge at least one offspring. For reasons outlined below, scientists usually prefer to estimate daily nest survival using data from participants who report data for each visit they make to the nest.

Accurately estimating nest success 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 more 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, most analysts use widely accepted methods to achieve accurate estimates of nest survival. Most modern methods are based on daily nest survival (i.e., the proportion of nests that survived from one period to the next, while the nests were under observation). The total number of nest-days of observation is called exposure days. In order for researchers to use exposure-based methods of analysis to estimate nest survival, we need to have a chronological record of each visit to the nest, which are reported as the nest visits.

Modern analyses will also allow researchers to examine how factors such as habitat, seasonal effects, or experimental treatments affect the probability of nest success.