Type: report

Article abstract: Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31 August 2001. Twenty-one transient flycatchers of unknown subspecies were detected during surveys. Transients occurred in a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian, willow-oak dominated riparian, and mule fat scrub. The distance from transient locations to the nearest surface water averaged 124  127 m (N = 21). Nineteen southwestern willow flycatcher breeding territories were located. With the exception of one territory at Lake O=Neill on Fallbrook Creek, all territories were along the lower Santa Margarita River. Ninety-five percent (18/19) of territories were located in mixed willow riparian habitat. Exotic vegetation, particularly poison hemlock (Conium maculatum) and giant reed (Arundo donax), was present in all but one territory, and was dominant (% cover > 50) in 32% (6/19) of territories. Resident flycatchers exhibited a bimodal distribution with regard to distance to surface water, with 42% within 60 m, and the remainder 150-500 m away from it. The resident flycatcher population included one unpaired male and 18 pairs (16 males, 18 females). Two males were believed to be polygynous, each mating with two females. Nesting was documented for 17 of the 18 pairs, which produced 1-3 nests each. Fifty-two percent (15/29) of nests were successful, and flycatchers fledged an average of 1.9 young per pair. No instances of cowbird parasitism were observed. Pairs placed nests in seven species of plants, including black willow (Salix gooddingii), arroyo willow (S. lasiolepis), stinging nettle (Urtica dioica), mule fat (Baccharis glutinosa), elderberry (Sambucus mexicana), poison hemlock, and giant reed. One resident male and one female were birds banded previously at Camp Pendleton; the male was banded as an adult in 1998, while the female was banded as an adult in 2000. Nine resident males and eight females were captured and color banded in 2001, and 26 nestlings in 12 nests were banded. None of the transients observed during surveys carried bands.