This is a good one, thanks @OllieClarke for help with debugging!
Turned out some of my issues were due to considering rabbits aged 4 months to be already in reproductive age, once I changed that, I got the right month, although my numbers are a bit off...
Another issue was the wrong data type which ended up with loads of nulls!
Struggled initially because I was interpreting it as >=4 rather than >4 for the age at which rabbits start producing! Definitely not the most efficient solution as it took 5 mins to run, but good iterative macro practice nonetheless!