Male and female trumpeter swans, called cobs and pens respectively, are visually very similar, with both having pure white plumage. However, cobs are generally larger, with a longer head and body, and a wider wingspan. The black knob at the base of the male's bill also tends to be more prominent, especially during the breeding season. [1, 1, 2, 2, 3, 4]
Here's a more detailed breakdown:
Appearance:
• Size and Weight: Cobs are typically larger than pens, with males exceeding 35 lbs and females ranging from 21-30 lbs.
• Plumage: Both sexes have pure white plumage, but the head and neck can sometimes appear stained orange or rust-colored from foraging in iron-rich wetlands.
• Bill: The bill is black and can sometimes show a salmon or orangish streak on the upper mandible. The bill is also longer and straighter than the similar tundra swan.
• Feet and Legs: The feet and legs are black.
• Young: Young swans, called cygnets, have light gray feathers.

Behavior:
• Pair Formation: Trumpeter swans form lifelong pairs, typically establishing their bond at around three years of age.
• Nesting: Both the male and female gather nesting material, usually marsh plants, and the female incubates the eggs.
• Territoriality: Trumpeter swans can exhibit aggressive behavior, particularly towards other pairs, especially during the breeding season.