ABSTRACT

I report on the strength of reversed sexual size dimorphism (RSD) in a local sub-population of Northern Pygmy-Owls (Glaucidium gnoma) in northwestern Oregon (2007–2012) in comparison to range-wide museum specimens examined by Earhart and Johnson (1970). Using Storer's Dimorphism Index (DI), RSD was higher among local breeding pairs (DImass = 6.64, DIwing = 4.90, n = 20 pairs) and among all local specimens (DImass = 6.20, DIwing = 4.39, n = 31 females, n = 23 males) than for specimens range-wide (DImass = 5.45, DIwing = 4.30, n = 10 females, n = 42 males). RSD was also more distinct locally, because mass alone was diagnostic of the sexes at a threshold of 69 g and wing chord was nearly diagnostic, whereas range-wide there was greater intrasexual variation and intersexual overlap in both metrics. Locally, females weighed 75.3 ± 5.6 g (range 69.0–94.2 g), and males weighed 62.5 ± 2.5 g (range 58.5–68.5 g); all local males had wing chords <92 mm (89.2 ± 2.0 mm; range 85.0–91.5 mm), whereas for 90% of females wing chord was ≥92 mm (93.2 ± 1.3 mm; range 89.0–96.0 mm). Clutch size (F1,4 = 1.822, P = 0.248, n = 20) and the number of fledged young (F1,8 = 0.00, P = 0.619, n = 19) were independent of RSD among local breeding pairs. Vertebrate prey composition shifted seasonally and was similar at the 2 scales, with agile avian prey taken most frequently during the breeding season, when the evolutionary consequences of diet on RSD matter most. I found intersexual variation in prey composition at the local scale where males took avian prey more frequently than females. I concluded that RSD in pygmy-owls was stronger at the local scale and that intersexual differences in prey composition may be related to RSD in this species. This study highlights the value of examining multiple scales and seasonal variation in prey composition when evaluating the merits of RSD hypotheses. It also contributes to our understanding of one of the least studied owls in North America.

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