Counting migrants to monitor bird populations: state of the art
This article is part of a larger document. View the larger document here.Abstract
The aim of this paper is to summarize background information on what migration monitoring is, what biases are present and how they can be addressed, evidence that resulting trends are biologically meaningful, and what the benefits and limitations of the method are. Some topics have been covered elsewhere in greater detail (Dunn and Hussell 1995, Dunn et al. 1997, Francis and Hussell 1998). However, an overview provides a useful introduction to this session, and here I discuss potential biases more fully than elsewhere. For the purposes of this presentation, "migration monitoring" refers to intensive (near daily) counting of migrants at specific sites, with the aim of tracking population change over time. Each daily count is a sample of the birds resting in, or passing through or over the specified count area within a 24-hr period. Counts can consist of many sample types, such as number of birds crossing the face of the moon, registrations on a radar screen, number of migrants recorded giving flight calls as they pass overhead, number of birds counted in diurnal migration or stopover, or number of resting birds captured in mist nets, to name a few. Here I concentrate on the latter two types; specifically, hawk watches and counts of nocturnal songbird migrants during stopover. In total, there are more than 150 North American landbird species for which migration counts are potentially as good as, or better than, other sources of data currently available for trend analysis (Hussell 1997, Dunn unpubl. data). These include raptors (which are often too sparsely distributed and secretive to be well sampled by the Breeding Bird Survey), long-distance migrants which breed in the boreal forest and winter in the neotropics (escaping coverage both by Breeding Bird Surveys [BBS] and Christmas Bird Counts), and short-distance migrants that breed in arctic and boreal zones and winter in the US.