Measurements

Variation is mainly between morphs and sexes. Morph is readily determined by plumage in the breeding season (May through August), and with some difficulty in basic plumage (Piper and Wiley 1990a).

With appropriate facilities both morph and sex can be determined accurately using DNA (Griffiths et al. 1998, Micholopoulos et al. 2007). During the breeding season (May-Aug) live birds can be sexed by cloacal or brood patch examination (Wolfson 1952). Laparotomy is possible year round. Museum specimens are assumed to have been sexed by dissection. Sexing of live birds at migration stations has been based mainly on measurement of the unflattened wing chord, which averages larger in males. However, there is a broad region of overlap, giving rise to two different but not completely satisfactory solutions. One avoids the overlap; the North American Bird Banding Manual (Pyle 1997) advises that birds with wing chord ≤ 67 mm are female, while those measuring ≥ 74mm are male. This conservative approach is accurate but leaves about two-thirds of birds unsexed. At the opposite extreme, birds with wing chords ≤ 70 mm are recognized as female, while those measuring > 70 mm are male; all birds are included but about 10% are assigned to the wrong sex (Piper and Wiley 1990a, Mazerolle and Hobson 2007). Intermediate scores have been used, sometimes using laparotomy to sex birds in the region of overlap (Atkinson and Ralph 1980).

Caldwell and Mills (2006) used spring birds in the Royal Ontario Museum collection, that could be assigned to morph (WS birds are on average larger than TS birds), to obtain data that narrowed the region of overlap between sexes if morph is known. Applied to wild birds, these measurements reduced unsexed individuals to about one-third. Combining several measurements and categorical variables in a multiple logistic regression model (MLR) achieved a very modest gain in ability to sex White-throated Sparrows (Schlinger and Adler 1990). Older data from studies that recognized WS birds as males and TS birds as females are difficult to interpret.

The following measurements from a variety of sources with reliable sexing include means, ranges, standard deviations or standard errors, and sample sizes as available. Data are for adults (see also Breeding: young birds for different stages of growth and development).

Linear

Bill length (exposed culmen). Adult male mean 11.4 mm, range 10.7-12.2, n=7; female 11.4, 11.2-11.7, n=7. No sex difference (Ridgway 1901).

Wing length (flattened wing chord). Male 73.9 mm, 72.0-77.8; female 69.3, 65.4-73.9 Male about 6% longer (Godfrey 1986).

WS male 73.3±0.4 SE, n=99; WS females 68.9±0.6, n=46; TS male 71.8±0.3, n=73; TS females 67.6±0.7, n=55. Males about 6% longer than female; in both sexes WS birds about 2% longer than TS birds (Caldwell and Mills 2006).

Breeding adults, Bradley, Maine. WS male 73.9±0.2, n=72; WS female 69.2±0.2, n=58; TS male 72.4±0.2, n=60; TS female 68.4±0.2, n=92. Significant effect of morph/sex (B. Horton, unpubl. data).

Wintering birds, Atlanta, Georgia. WS male 73.8, 70-79, n=58; TS male 72.9, 70-78, n=59. Difference significant p=0.03. WS female 69.2, 65-73, n=103; TS female 69.0, 66-73, n=105 (D. Maney, unpubl. data).

Tail length. Male 73.2 mm, 71.1-76.2, n=7; female 69.6, 68-173.7, n=7. Male approximately 4% longer (Ridgway 1901).

Tarsus length. Male 23.6 mm, 22.9-24.6, n=7; female 23.1, 22.3-23.9, n=7. Male about 2% longer (Ridgway 1901).

Breeding adults: Algonquin Park 24.2 ± 0.8, n=31 (Kopachena 1992). Bradley, Maine. WS male 23.64±0.08, n=72; TS male 22.26±0.06, n=60; WS female 23.10±0.06, n=58; TS female 22.71±0.07, n=93. Significant effect of morph/sex (B. Horton, unpubl. data). See also Breeding: Young Birds.

Longest toe (length of third digit from separation of second digit, without claw). Male 16.8mm, 16.3-17.3, n=7; female 16.0, 15.2-16.5, n=7. Male about 5% longer (Ridgway 1901). Wintering birds, Atlanta, Georgia. Right foot, male 15.9, 15.0-16.8, n=26; female 15.2, 14.1-16.4, n=38. Left foot, male 15.6, 14.5-16.6, n=26; female 15.0, 13.6-16.6, n=38 (D. Maney, unpubl. data).

Bill, wing, and leg morphology correlate with crown morph (Rising and Shields 1980). TS males have longer, narrower bills, shorter wing elements, shallower keel, and stouter legs than WS males. TS females are smaller than WS females for all characters except culmen width. These authors suggest differences may be related to habitat partitioning (see also Tuttle 1993).

Mass

Both sexes 25.9 g ± 2.2 SD, 19.0-35.4, n=1884 (Dunning 1993). Varies seasonally. Males 5-14% heavier than females.

Breeding adults, Adirondack Park, NY. WS male 26.4 g ± 1.7 SD, n=34; TS male 25.2±1.8, n=20; WS female 25.1±2.2, n=19; TS female 24.1±1.4, n=23 (Tuttle 1993). Bradley, Maine. WS male 25.8±0.2 SE, n=70; TS male 24.9±0.2, n=58; WS female 25.0±0.2, n=58; TS female 24.4±0.2, n=90 (B. Horton, unpubl. data).

Overwinter, based on wild birds trapped or shot during late afternoons in n. Georgia. Postmigration (Oct-Nov) male 27.3 g, 23.1-28.9, n=12; female 25.3 g, 22.5-28.3, n=11. Midwinter (Jan-Feb) male 31.4, 28.4-35.4, n=9; female 28.4, 27.1-30.4, n=6. Molt (Mar-Apr) male 28.1, 26.4-32.0, n=18; female 26.1, 23.1-28.2, n=9. Premigration (late Apr) male 32.8, 29.7-36.1, n=7; female 28.1, 23.2-31.4, n=12 (Odum and Perkinson 1951). Similar trends summarized for several wintering areas (Prescott 1986). Athens, Georgia. WS male 26.1, 23-31, n=47; TS male 25.6, 23-28.5, n=53; WS female 23.5, 21-27.5, n=93; TS female 23.4, 20-28, n=95 (D. Maney, unpubl. data).

Because of difficulty of sexing birds of intermediate size (see above), masses of migrants are not given. Masses generally greater in spring than in fall, presumably because greater fat deposition is needed for more rapid migration and greater demands on arrival in spring (J. D. McCracken, based on unpubl. data from Long Point, Ontario). In New Jersey, fall migrants are heavier with more fat at inland sites than on the coast; difference attributed to long overwater migration before reaching the coast (Prescott 1986).

Tarsus length and wing chord correlate with mass and are often used to correct mass for body size (e.g. as a measure of “body condition”). The significant effect of morph/sex on both these measurements should be considered when making these corrections (B. Horton pers. comm.).

For factors affecting mass see Migration: Control and Physiology, Food Habits: Metabolism and Temperature Regulation.