A tūī in silhouette

This tūī (Prosthemadera novaeseelandiae) in silhouette shows off several gaps in its wings. The outermost feathers on its wings are the primaries and they’re labelled from the centre of its body outward. The 8th primary feather of the tūī (the third from the outer edge) has a gap, while primaries 7 to 4 each have a bulge, which you can also see in this photo. The notch in the 8th primary is responsible for the whirring noise you often hear when a tūī flies overhead, especially when there’s more than one engaged in a tūī dogfight.