GB- Four birds on one cover

From my good friend in Birmingham comes this neat arrangement of four birds on one cover.
First is the 1977  9p Grey Partridge (Perdix perdix), next to the elegant  2007 Pied Avocet   ( Recurvirostra avosetta). There's a 30 year age gap between these two !!!. Below them are a pair from the
1980 Wild Bird Protection series , White- throated dipper (Cinclus cinclus) and the Common Kingfisher (Alcedo atthis)