Swans » Bewick`s Swan
Bewick`s Swan (Cygnus bewickii)
Bewick"s Swans are the smaller subspecies. There is a slight size cline, with the eastern birds being slightly larger; good measurement data only exists for the western populations however.
These weigh 3.4–7.8 kg (7.5–17.2 lb), 6.4 kg (14 lb) on average in males and 5.7 kg (13 lb) in females. They measure 115–140 cm (45–55 in) in overall length; each wing is 46.9–54.8 cm (18.5–21.6 in) long, on average 51.9 cm (20.4 in) in males and 50.4 cm (19.8 in) in females.
The tarsus measures 9.2–11.6 cm (3.6–4.6 in) in length, the bill 8.2–10.2 cm (3.2–4.0 in), averaging 9.1 cm (3.6 in). Bewick"s Swan is similar in appearance to the parapatric Whooper Swan (C. cygnus), but is smaller, shorter-necked and has a more rounded head shape, with variable bill pattern, but always showing more black than yellow and having a blunt forward edge of the yellow base patch.
Whooper Swans have a bill that has more yellow than black and the forward edge of the yellow patch is usually pointed. The bill pattern for every individual Bewick"s Swan is unique, and scientists often make detailed drawings of each bill and assign names to the swans to assist with studying these birds.
The eastern birds, apart from being larger, tend towards less yellow on the bill, perhaps indicating that gene flow across Beringia, while marginal, never entirely ceased. An apparent case of hybridization between a Bewick"s and a vagrant Whistling Swan has been reported from eastern Siberia.
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