Birds are an incredibly diverse animal group. This is reflected in many aspects of their reproductive ecology, as you can see in this short list of facts about the nests of birds.

Largest tree nest: a Bald Eagle nest in Florida which was 20 feet (6.1m) deep, 9.5 feet (2.9m) wide, and weighed almost 6000 pounds (2,722 kg).

Largest ground nest: a Dusky Scrubfowl nest, which was 36 feet (11m) wide, 16 feet (4.9 m) high and incorporated over 300 tons (2,700 kg) of forest floor litter.

Largest social nest:  the African Social Weavers, with a 100 chamber nest structure that measured 27 feet (8.2 m) in length and 6 feet (1.8 m) high.

Largest roofed nest: a Hamerkop nest that was 6.5 feet (2.0 m) wide and 6.5 feet (2.0 m) deep.

Smallest nest:  the Cuban Bee Hummingbird and Vervain Hummingbird; their nest is a mere  0.78 inches (1.98cm) in breadth and 0.78-1.2 inches (1.98- 3.0 cm) deep.

Highest tree nest: a Marbled Murrelet nest located 148 feet (45m) above the ground.

Longest underground nest burrow: a Rhinoceros Auklet nest, located at the end of a 26 feet (8m) long burrow.

Largest recorded nesting bird colony: a Passenger Pigeon colony with an estimated 136 million nests, which covered about 750 square miles (1942 square kilometres).

Most northerly nesting bird:  the Ivory Gull, who nests on isolated mountain peaks at the edge of permanent pack ice, far to the north of the Arctic Circle.

Source of these facts: The Bird Almanac, by Dr. David M. Bird