This past month, especially, this area of the Cashmere Hills has seen lots of black shags (plus a few pied shags) flying by, almost always heading southwest or north-northeast. It must be the current flight path between the Avon-Heathcote estuary and Lake Ellesmere.

(My manual focus telephoto lens means some photos like this are annoyingly blurry. It should still be sufficient to confirm the ID.)