THE HIDDEN HOMES OF BANK SWALLOWS

These birds burrow into vertical silt or sand banks

1 COLONIES

These birds breed in large nests that can house hundreds of pairs of swallows.

2 NEST SITES

As well as sand banks, these birds make homes in cliffs, riverbanks and wall drain pipes.

3 CREATING TUNNELS

Birds dig into the loose ground with their beaks to form 45 to 90 centimetre long tunnels, with chambers at the end of each.

4 TEAMWORK

Both male and female bank swallows take part in the nest-burrowing process.

5 TAKEOVER

If the burrows are built too wide, birds such as sparrows and starlings can take over the home.

6 INCUBATION

Four or five eggs are usually laid per chamber in May or June. They hatch after two weeks, but remain in the nest for around 20 days after hatching.