The damage dealt by the birds may not always be the same or consistent, and it can be influenced by various factors in the PVP mode.
Note that the bird's damage may be influenced by runes equipped on it, therefore, the higher the rune tier the better. Critical damage that a bird deals is a bit higher in comparison to regular attack. Another factor is that if a bird on your team has been upgraded even a little, the power of pigs has been adjusted automatically to match yours and the opponent's strength, and this is why slightly less damage has been dealt compared to what you have gotten used to.