What symbolic act did the animals perform as soon as Farmer Jones and all the other humans had fled the farm?

From the text:

An impromptu celebration of the victory was held immediately. The flag was run up and 'Beasts of England' was sung a number of times, then the sheep who had been killed was given a solemn funeral, a hawthorn bush being planted on her grave. At the graveside Snowball made a little speech, emphasising the need for all animals to be ready to die for Animal Farm if need be.