The lyrics are so varied from version to version, depending on how, um, nasty, one could afford to be!  
Here is a longish version of the lyrics.  Then we'll talk about the variations.  
(opening verse)
When the little bluebird 
 Who has never said a word 
 Starts to sing "Spring, spring"
 When the little bluebell 
 At the bottom of the dell 
Starts to ring, ding ding
 When the little blue clerk 
 In the middle of his work 
 Starts a tune to the moon up above 
 It is nature, that's all 
Simply telling us to fall in love 
And that's why birds do it, bees do it 
Even educated fleas do it 
Let's do it, let's fall in love 
In Spain the best upper sets do it
 Lithuanians and Letts do it
 Let's do it, let's fall in love 
 The Dutch in old Amsterdam do it
 Not to mention the Finns
 Folks in Siam do it--
 Think of Siamese twins.
 Some Argentines, without means, do it 
 People say in Boston even beans do it 
 Let's do it, let's fall in love 
 Romantic sponges, they say, do it
 Oysters down in Oyster Bay do it
 Let's do it, let's fall in love
 Cold Cape Cod clams, 'gainst their wish, do it 
 Even lazy jellyfish do it 
 Let's do it, let's fall in love 
 Electric eels, I might add, do it,
 Though it shocks 'em, I know,
 Why ask if shad do it?
 Waiter, bring me shad roe!
 In shallow shoals, English soles do it
 Goldfish in the privacy of bowls do it
 Let's do it, let's fall in love
(the following are more rarely performed)
 Mosquitos, heaven forbid, do it
 So does every katydid do it
 Let's do it, let's fall in love
 Dragonflies in the reeds do it
 Sentimental centipedes do it
 Let's do it, let's fall in love
 The most refined ladybugs do it 
 When a gentleman calls 
 Moths in your rugs do it 
 What's the use of moth balls?
 The locusts up in the trees do it
 Even over-educated fleas do it
 Let's do it, let's fall in love
 The chimpanzees in the zoos do it
 Some courageous kangaroos do it
 Let's do it, let's fall in love
 I'm sure giraffes on the sly do it
 Heavy hippopotami do it
 Let's do it, let's fall in love
 Sloths who hang down from the twigs do it
 Though the effort is great
Sweet guinea pigs do it-- 
 Buy a couple and wait!
 The world admits bears in pits do it
 Even Pekingeses in the Ritz do it
 Let's do it, let's fall in love