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 15 August 2007. Seventy transient flycatchers of unknown sub-species were observed during Basewide surveys. Transients occurred on 12 of the 16 drainages surveyed in 2007. No willow flycatchers were detected at De Luz, Horno, Roblar, or Windmill Creeks. Transients occurred in a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian, oak-sycamore dominated riparian, riparian scrub, and upland scrub. The distance from transient locations to the nearest surface water averaged 340 ± 424 m (std, n = 70). In 2007, the resident southwestern willow flycatcher population on Base consisted of 14 females and 12 males. However, because of within season flycatcher movement, 16 territories were established. One male defended territories in two locations, separated by more than 1 km, pairing with a female in the second location. Another male remained single during the entire 2007 breeding season. In total, 14 females formed pair bonds with 11 male willow flycatchers. Two of the 11 paired males were polygynous with two females each. Based on movement data, two additional males were suspected to be polygynous with neighboring females. All territories were located in mixed willow riparian habitat. Poison hemlock (Conium maculatum) was present in all territories. Distance to surface water averaged 168 ± 244 m (std, n = 16), with 69% (11/16) of territories located within 100 m of water. Nineteen nesting attempts by willow flycatchers were documented during the 2007 breeding season. Nesting was initiated in early June and continued into August. Forty-two percent (8/19) of nests successfully fledged at least one flycatcher young. Predation accounted for 73% (8/11) of nest failures. The other documented cause of nest lost was substrate failure. The cause of failure for two nests was unknown. It is possible that they were depredated in the egg stage or abandoned prior to egg laying, as they failed during the time eggs should have been laid, but no eggs were observed in the nests. Of the 12 pairs whose nests were monitored, 67% (8/12) fledged young. Seventeen fledglings were produced, yielding an estimated seasonal productivity of 1.4 young per pair (17 young/12 pairs). No instances of brown-headed cowbird (Molothrus ater) parasitism were observed. P