Southwestern willow flycatcher surveys, demography, and ecology along the lower Colorado River and tributaries, 2006

McLeod, M.; Koronkiewicz, T.; Brown, B.; Carothers, S.

Reclamation initiated willow flycatcher studies along the lower Colorado River in 1996, in anticipation of the requirements outlined in the BOs that were part of MSCP development. These studies have been conducted annually since 1996. In compliance with the consultation on Interim Surplus Criteria and Secretarial Implementation Agreements, Reclamation biologists deployed temperature/humidity data loggers in 2004 at a subset of sites currently monitored for Southwestern Willow Flycatcher along the Colorado River in California and Arizona. These studies were expanded in 2005 to include monitoring of groundwater levels, vegetation, and soil moisture in addition to temperature and humidity... INTRODUCTION Broadcasts of recorded conspecific vocalizations are useful in eliciting responses from nearby willow flycatchers, and multiple broadcast surveys conducted throughout the breeding season are the standard technique for determining the presence or absence of E. t. extimus (Sogge et al. 1997). Willow flycatchers detected between approximately 15 June and 20 July in the breeding range of E. t. extimus probably belong to the southwestern subspecies (Sogge et al. 1997, USFWS 2002). However, because northbound individuals of all subspecies of the willow flycatcher migrate through areas where E. t. extimus are actively nesting, and southbound migrants occur where extimus are still breeding (USFWS 2002, Sogge et al. 1997), field confirmation of the southwestern subspecies is problematic.1 For example, the northwestern E. t. brewsteri, far more numerous than E. t. extimus, has been documented migrating north in southern California as late as 20 June (Garrett and Dunn 1981 as cited in Unitt 1987), and Phillips et al. (1964 as cited in Unitt 1987) documented E. t. brewsteri collected in southern Arizona on 23 June. An understanding of willow flycatcher migration ecology in combination with multiple broadcast surveys conducted throughout the breeding season is therefore needed to assess the presence and residency of Southwestern Willow Flycatchers. Migration routes used by E. t. extimus are not well documented, though more is known of northbound migration in spring than the southbound migration in fall because spring is the only time that migrant willow flycatchers sing and can therefore be distinguished from other Empidonax species. During northbound migration, all subspecies of willow flycatchers use riparian habitats similar to breeding habitat along major river drainages in the Southwest such as the Rio Grande (Finch and Kelly 1999), Colorado River (McKernan and Braden 1999), San Juan River (Johnson and Sogge 1997), and the Green River (M. Johnson unpubl. data). Although migrating willow flycatchers may favor young, native willow habitats (Yong and Finch 1997), migrants are also found in a variety of unsuitable breeding habitats in both spring and fall. These migration stopover habitats, even though not used for breeding, are likely important for both reproduction and survival. For most long-distance Neotropical migrant passerines, migration stopover habitats are needed to replenish energy reserves to continue northbound or southbound migration.

Southwestern willow flycatcher surveys, demography, and ecology along the lower Colorado River and tributaries, 2006

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