Estimating nests not present at the time of breeding surveys: an important consideration in assessing nesting populations

Abstract

Counts of nest starts are often used as indicators of the size of avian nesting populations, or of avian productivity. However, the accuracy of single or repeated counts of unmarked nests over time for estimating seasonal numbers of nests may be strongly affected by nest events that fall in between survey dates, or that occurred prior to or after the survey period. Accuracy may also be affected by uncertainty in the interpretation of counts due to overlap between starting and ending dates of asynchronous nests during the intervisit interval. To measure the combined magnitude of these effects on survey accuracy, we overlaid a monthly “survey” regime on known initiation and ending dates of 2055 nests of ciconiiform birds. Assuming all nests present on the date of simulated survey were counted, monthly surveys underestimated the true number of nest starts by 24–64%, depending on species and year. Using a simple model, we also demonstrate that accuracy does not increase much as survey frequency increases, and that significant estimation error can occur over a wide range of nest success values and degrees of asynchrony. We suggest that (1) these biases can be significant for surveys of many kinds of nesting birds including some territorial passerines, (2) this bias cannot be addressed by increasing survey frequency, and (3) the degree of renesting may be of critical interest for inferring breeding population size from nest count data. We suggest three possible approaches for modeling this error.

Use of the superpopulation approach to estimate breeding population size: an example in asynchronously breeding birds

“…Variation in error rates was much larger between years and between colonies during the same year than between species (Table 1; see also Frederick et al 2006). This suggests that factors affecting nest success and asynchrony are quite variable over time and/or space.…”

“…Most studies of estimation error to date have focused on estimating animals that are present at the time of survey, and have only rarely recognized the problem of estimating animals that are present at some point during the period of interest but that are not present at the time of survey (Frederick et al 2006). This latter issue can arise when individuals are not individually identifiable, and they join and leave surveyed populations at an unknown frequency.…”

“…This latter issue can arise when individuals are not individually identifiable, and they join and leave surveyed populations at an unknown frequency. This problem has been recognized in colonially breeding birds (Frederick et al 2006), ungulates (Gould et al 2005), and spawning fishes (Schwarz et al 1993), among other taxa, as well as being a common circumstance at migration stopover sites (Williams et al 2002).…”

“…This is true even if the counts on each date are highly accurate. While the resulting bias may be negligible when aggregation or reproduction is highly synchronous, the degree of error in less synchronous species can be substantial (e.g., 47% in breeding wading birds [Frederick et al 2006], 69-79% in shorebird populations at stopover sites during migration [Farmer and Durbian 2006]). …”

“…However, it is rare that entire breeding populations can be counted directly, and most attempts at estimating population size are approximations derived from incomplete surveys with various potential sources of bias (Williams et al 2002, Frederick et al 2006.…”