The smallest birb code that reduces to Church numeral n:

0: 2 birbs
🐥🐦
🦩🐥
🦢🐥

1: 2 birbs
🦚🐦
🐧🐦

2: 3 birbs
🦢🦚🐦
🦢🐧🐦

3: 6 birbs
🦜🦩🕊🦢🦚🐦
🦜🦩🕊🦢🐧🐦
🦜🦩🦉🦢🐧🐦
🦜🦩🪽🦢🕊🐦

4: 4 birbs
🦜🦢🦚🐦
🦜🦢🐧🐦

...