Southwestern Willow Flycatcher Breeding Site and Territory Summary - 2007

Abstract

The Southwestern willow flycatcher (Empidonax traillii extimus; hereafter references to willow flycatcher and flycatcher refer to E.t. extimus, except where specifically noted) is an endangered bird that breeds only in dense riparian habitats in parts of six Southwestern states (Arizona, New Mexico, southern California, extreme southern Nevada, southern Utah, and southwestern Colorado). Since 1993, hundreds of Southwestern willow flycatcher surveys have been conducted each year, and many new flycatcher breeding sites located. This document synthesizes the most current information available on all known Southwestern willow flycatcher breeding sites. This rangewide data synthesis was designed to meet two objectives: (1) identify all known Southwestern willow flycatcher breeding sites and (2) assemble data to estimate population size, location, habitat, and other information for all breeding sites, for as many years as possible, from 1993 through 2007. This report provides data summaries in terms of the number of flycatcher sites and the number of territories. When interpreting and using this information, it must be kept in mind that a 'site' is a geographic location where one or more willow flycatchers establishes a territory. Sites with unpaired territorial males are considered breeding sites, even if no nesting attempts were documented. A site is often a discrete patch of riparian habitat but may also be a cluster of riparian patches; there is no standardized definition for site, and its use varies within and among states. For example, five occupied habitat patches along a 10-km stretch of river might be considered five different sites in one state but only a single site in another state. This lack of standardization makes comparisons based on site numbers problematic. Researchers for this report generally deferred to statewide summary documents or to local managers and researchers when delineating a site for inclusion in the database. However, to avoid inflating the number of sites and to establish more consistent definitions of the term 'site', adjacent and nearby sites from statewide reports were sometimes considered as a single site for the purposes of the rangewide data summary. Any combining or splitting of sites at the rangewide level was done on a case-by-case basis. Because of differences in site definitions, one should not evaluate the relative importance of a geographic region (such as drainage, watershed, or state) simply according to the number of flycatcher sites. A 'territory' is an exclusive defended area within a breeding site. Although detailed monitoring studies have identified unpaired territorial males and polygynous males at some flycatcher breeding sites, for the purposes of this report a territory is equivalent to the exclusive breeding area of a pair of flycatchers. In general, the concept of territory is more similar among states and different investigators than site; thus, it is a more robust unit to use for summaries and comparisons. However, note that the definition of a polygynous territory is not consistent among states; a male polygynously paired with two females would be considered one territory in some states and two territories in other states. For each site, we referred to reports or spoke directly with researchers and managers to gather information such as management entity/agency, location (state, drainage, elevation), gross habitat type (native, exotic, or mixed; dominant tree species), and number of flycatcher territories. Synthesizing the information on more than 200 breeding sites is challenging because annual data-collection and survey-reporting requirements are not standardized rangewide, and the nature and degree of readily available information varies widely from state to state. This is particularly true for areas such as California, where there are many flycatcher sites but surveyors are not required to submit standardized flycatcher survey forms. The lack of c