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 lb), 6.4 kilograms (14 lb) on average in males and 5.7 kilograms (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 centimetres (20.4 in) in males and 50.4 centimetres (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 centimetres (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.
In summer, their diet consists mainly of aquatic vegetation – e.g. mannagrass (Glyceria), Potamogeton pondweeds and marine eelgrass (Zostera), acquired by sticking the head underwater or upending while swimming; they also eat some grass growing on dry land. At other times of year, leftover grains and other crops such as potatoes, picked up in open fields after harvest, make up much of their diet. Tundra Swans forage mainly by day. In the breeding season, they tend to be territorial and are aggressive to many animals who pass by; outside the breeding season they are rather gregarious birds.
Healthy adult birds have few natural predators. Arctic Foxes (Vulpes lagopus) may threaten breeding females and particular the eggs and hatchlings; they can be quite hard to scare away. About 15% of the adults die each year from various causes, and thus the average lifespan in the wild is about 10 years. The oldest recorded Tundra Swan was over 24 years old.
The Tundra Swans mate in the late spring, usually after they have returned to the nesting grounds; as usual for swans, they pair monogamously until one partner dies. Should one partner die long before the other, the surviving bird often will not mate again for some years, or even for its entire life. The nesting season starts at the end of May. The pair build the large mound-shaped nest from plant material at an elevated site near open water, and defend a large territory around it. The pen (female) lays and incubates a clutch of 2-7 (usually 3-5) eggs, watching for danger while sitting on the nest. The cob (male) keeps a steady lookout for potential predators heading towards his mate and offspring. When either of them spots a threat, they give a warning sound to let their partner know that danger is approaching. Sometimes the cob will use his wings to run faster and appear larger in order to scare away a predator.
The time from laying to hatching is 29–30 days for Bewick's Swan and 30–32 days for the Whistling Swan. Since they nest in cold regions, Tundra Swan cygnets grow faster than those of swans breeding in warmer climates; those of the Whistling Swan take about 60–75 days to fledge – twice as fast as those of the Mute Swan for example – while those of Bewick's Swan, about which little breeding data is known, may fledge a record 40–45 days after hatching already. The fledglings stay with their parents for the first winter migration. The family is sometimes even joined by their offspring from previous breeding seasons while on the wintering grounds; Tundra Swans do not reach sexual maturity until 3 or 4 years of age.
The Whistling Swan is the most common swan species of North America, estimated to number almost 170,000 individuals around 1990. Its numbers seem to be slowly declining in the west of its range since the late 19th century, coincident with the expansion of human settlement and habitat conversion in the birds' wintering areas; the estern Whistling Swan populations on the other hand seem to be increasing somewhat, and altogether its numbers seem to have slightly risen in the late 20th century (the population was estimated at about 146,000 in 1972). Bewick's Swan remains far less known; the European winter population was estimated at 16,000-17,000 about 1990, with about 20,000 birds wintering in East Asia. The Iranian wintering population is small – 1,000 birds or so at most – but they usually disperse to several sites, some of which are still unknown to scientists.
Although Tundra Swan numbers are stable over most of its range, they are increasingly dependent on agricultural crops to supplement their winter diet, as aquatic vegetation in their winter habitat dwindles due to habitat destruction and water pollution. But the main cause of adult mortality is hunting; 4,000 Whistling Swans are bagged officially each year, while a further 6,000-10,000 are killed by poachers and native subsistence hunter-gatherers. Bewick's Swan cannot be hunted legally, but almost half the birds studied contained lead shot in their body, indicating they were shot at by poachers. Lead poisoning by ingestion of lead shot is a very significant cause of mortality also, particularly in the Whistling Swan. The Tundra Swan is not considered threatened by the IUCN due to its large range and population. The proposed subspecies jankowskii was for some time placed on CITES Appendix II; it was eventually removed since it is not generally accepted as valid.