Many applied and theoretical investigations require information on how productivity varies in time and space (Temple and Wiens 1989. DeSante 1995). Examples include studies of habitat quality, population trends, life-history tactics, and metapopulation dynamics. From a demographic perspective, productivity is the number of young counted at a given time of year, produced per adult (e.g. Caswell 1989). Various measures have been used to estimate productivity. One of the most attractive is mist netting during the summer after young have left the nest, but ideally before they have left the study area. Several programs use this approach, including the Constant Effort Sites Scheme of the British Trust for Ornithology (Baillie et al. 1986, Bibby et al. 1992) and the Monitoring Avian Productivity and Survivorship (MAPS) program (DeSante et al. 1993) in North America.
Hatching-year (HY) and after-hatching-year (AHY) birds are widely believed to have different susceptibilities to netting (DeSante et al. 1995, Peach et al. 1996), so the ratio of HY's to AHY's obtained from netting is not used as an estimate of productivity. Instead, investigators hope that the relative susceptibility to capture is about the same among the samples being compared so that the age ratios in mistnet samples provide a reliable index to productivity (DeSante 1995, DeSante et al. 1995).
