Preheat oven to 180°C and grease four 23cm cake tins.
Beat butters with sugar until fluffy and gradually add eggs, flour, zest, and baking powder.
Divide batter between tins and bake for 20-25 minutes before cooling.
Prepare a second batter and divide into three bowls colored black, yellow, and pink.
Bake the colored sponges in 8cm tins for 15-20 minutes and cool.
Whip butter and icing sugar with lemon juice and zest to create buttercream.
Shape the yellow sponge into a 5cm beak and the pink sponge into a nose.
Stack three lemon sponges with buttercream and cut a tapered cylinder cavity in the center.
Insert the beak and nose pieces into the cavity using buttercream to secure.
Add the final lemon cake layer and carve a 10cm wide dome cavity for the head.
Insert a shaped dome of black sponge into the top cavity.
Carve the cake edges to resemble a pie shape and coat the entire cake with buttercream.
Wrap the cake sides with white fondant and decorate with blue icing dots.
Apply cream fondant to the top, crimp the edges, and cut a cross in the center.
Place small fondant blackbird heads inside the top cross to finish.