Bird is a white bird who takes an interest in Willa between The Knights of the Wobbly Table and Don't Lead Me a Stray
It appears to be a seagull and takes the green tennis ball that Willa was playing with.
It appears again in the interstitial after Things That Go Bump.
Seagul is revealed to be its name in the captions of Sherlock Bones.
Mr. Seagull is what Bailey calls the bird, assuming a male gender.
Seagull is used in a subsequent caption, indicating the earlier 'Seagul' was a typo and not confirming Bailey's gender assumption.
It plays balloon with frog and Willa in ep 6 interstitial between Coming Soon! and Fire Dog Tucker