Measurements

See Appendix . Swarth (1920) gives mean values and ranges for samples of males of w. North America subspecies. Pyle (1997) gives ranges (without means or standard deviations) for males and females of most subspecies. Females slightly smaller than males: about 2–4 mm in wing-chord, 1–3 mm in tail, about 0.5 mm in bill (see Ridgway 1901, Rising 1996, Pyle 1997). P. i. iliaca males and females differ significantly in measurements of wing and tail (Cramp and Perrins 1994).

Within the iliaca group, populations vary clinally with northernmost populations (in Alaska) largest. Within unalaschcensis group, populations in Aleutians and sw. Alaska larger bodied and larger billed than those south of Yakutat Bay, AK (Swarth 1920, Rising 1996). Within schistacea group, measures of size (wing-chord, bill width, tarsus; also skeletal elements and mass) increase from P. i. schistacea populations of ne. Nevada to populations of P. i. canescens and P. i. swarthi of n. Utah (Swarth 1920, Zink 1986). Megarhyncha group also shows a north-to-south cline of increasing bill size (Swarth 1920, Zink and Kessen 1999).

See Appendix . For Alameda Co., CA, during non-breeding season, mean mass 32.4 g (range 21.7–42.1, n = 221 measures of 91 individuals of both sexes over period Sep 1932–May 1933); on average, mass increases from midmorning to evening; monthly means show a peak for Dec (34.0 g) and a premigration increase in Apr and May (35.6 g; Linsdale and Sumner 1934). For migrants in Pennsylvania, mean 36.9 g (range 29.6–49.0 [n = 446]; Clench and Leberman 1978). For spring and fall migrants in Cook, Co., IL, male mean 36.1 g ± 2.5 SD (range 24.9–44.9, n = 526), female mean 33.9 g ± 2.4 SD (range 25.3–45.7, n = 466; Field Museum of Natural History data from collision mortality).