Northern Pygmy-Owl (Glaucidium gnoma) nesting ecology in northwestern Oregon

Abstract

I report on the nesting ecology of the Northern Pygmy-Owl (Glaucidium gnoma) in northwestern Oregon based on 71 breeding attempts, including observations at 66 nest cavities during 12 years of study (2007–2013, 2016–2020). I focus on the annual shift in the start of laying and in productivity (clutch size) in relation to prey composition. For all years combined, the start of laying spanned 2 months (27 Mar–30 May). The mean annual start of laying varied across consecutive years by nearly 3 weeks (19.0 ± 6.5 d); mean productivity at nests varied annually by 23% (1.2 ± 0.5 eggs). Observed clutch size was most frequently 5 or 6 eggs (5.6 ± 0.9, range 4–8 eggs, n = 47), but most clutches in early-nesting years were 6 or 7 eggs, and 4 or 5 eggs in late-nesting years. The mean incubation and nestling periods were 29.4 ± 1.1 d and 26.4 ± 1.2 d, respectively. Pygmy-owls fledged a mean of 5.1 ± 1.1 young at 58 successful nests and 4.2 ± 2.2 for all breeding attempts. Vertebrate prey composition varied annually and seasonally. Pygmy-owls took mammalian prey more often early in the breeding season (66% of vertebrate prey) and in early-nesting years (65%); avian prey were more frequently taken late in the breeding season (66%) and in late-nesting years (66%). Pygmy-owls nested disproportionately in coniferous trees (χ2 = 67.48, P < 0.001, df = 1, n = 66); nests in western redcedars (Thuja plicata) fledged 1.5 more owlets and failed less often (5% failed) than nests in other tree species (22% failed). Observations at this largest ever collection of Northern Pygmy-Owl nests reveal a commitment by these small owls (1) to produce young annually, even when they had to overcome obstacles such as mate procurement, mate loss, and nest depredation, and (2) to use mammalian prey to increase productivity in some years.