Measures of productivity and survival derived from standardized mist-netting are consistent with observed population changes

Abstract

To investigate potential biases in productivity indices and survival-rate estimates from four years (1992–95) of data from the Monitoring Avian Productivity and Survivorship (MAPS) Programme, we examined correlations and regressions of trends in adult captures on population changes modelled from productivity indices and survival-rate estimates for target species at both local and regional scales. We estimated trends in adult captures from regressions of chain indices on year. We calculated mean numbers of young per adult, and estimated time-constant, annual adult survival rates from mark-recapture data using both non-transient and transient models. We used these productivity indices and survival-rate estimates in a simple demographic model to calculate predicted rates of population change. We modelled survival of young birds (ф0y ; from initial capture, generally during juvenile dispersal, to recruitment into the following year's breeding population) relative to adults (ф0a) as ф0y = 0.75 x ф0a. At the local scale, correlation coefficients between trends in adult captures and modelled population changes tended to be positive, but the strengths of the correlations were often weak. As expected, trends in adult captures generally were better correlated with modelled population changes than with either productivity or survival. Stronger correlations between trends in adult captures and modelled population changes tended to be found using survival estimates from the non-transient model, but modelled population changes using the transient model gave values for mean population changes closest to those derived from trends in adult captures. Modelled population changes sometimes were less well correlated with trends in adult captures at the regional than at the local scale, presumably because of spatial heterogeneity in the factors affecting population dynamics of target species. These results suggest that absolute measures of productivity derived from MAPS are likely to be biased, but do not show unequivocally the extent to which such biases might differ between species. Nevertheless, given the relatively low precision of analyses based on only a few years of data, these results suggest that relative measures of productivity and survival derived from MAPS do reflect observed population changes. Longer runs of data should allow the demographic mechanisms causing population changes to be investigated.