假设我们有这样的东西

df <- data.frame(species = c("Passer", "Turdus", "Turdus", "Gallus", "Anas", "Anas"),
                 season = c("breeding", "breeding", "non-breeding", "non-breeding", "breeding", "non-breeding"), 
                 value = seq(1, 12, 2), 
                 drop = c("no", "no", "dog", "dog", "cat", "cat"))

species       season value drop
1  Passer     breeding     1   no
2  Turdus     breeding     3   no
3  Turdus non-breeding     5  dog
4  Gallus non-breeding     7  dog
5    Anas     breeding     9  cat
6    Anas non-breeding    11  cat

我想选择具有繁殖季节两个值的物种的行，即繁殖和非繁殖。
结果应如下所示

species       season value drop
2  Turdus     breeding     3   no
3  Turdus non-breeding     5  dog
5    Anas     breeding     9  cat
6    Anas non-breeding    11  cat

我用filter（）试了几次，但是我不能让它工作。我怀疑做一个循环可能是一种方法？谢谢!