The Birthday Song

The Birthday Song is a song, whose actual title is unknown, sung by members of The Rats. The song plays at Michael's birthday party, because Michael is apparently a huge fan of the Rats.

The song can be personalized with the name of the person whose birthday it is, but this is not done particularly well.

Michael's father remarks that he "had to go through so much trouble" to get the song on (presumably) casette, implying that the Rats have a very large fanbase. The Rats also seem to have a mostly male fanbase, because the song specifically refers to its subject as a "boy". It is unknown if there is a female version.

The song seems to summon several Rats, including Giant Rat #9,375 to Michael's house. Perhaps the song has some effect on the Rats, or they are somehow able to tell when and where it is playing.

Lyrics
The lyrics are not entirely confirmed, but the general premise of them is widely known.

Rats, rats, we are the rats

Celebrate another, wanna birthday bash 

, it's your birthday today

Cake and ice cream is on its way

And  was such a good boy this year

Open up your gifts while we all cheer

Happy birthday,  !

The song may have more lyrics, but it is interrupted by the entry of The Clown.