Seabirds – Spot the difference

Some people tell me that all seabirds look alike and are hard to tell apart. Well, it’s true, some do, and even in the hand it can be very hard to tell the difference; especially these two – Pycroft’s and Cook’s petrels. They are distinct species and identification is everything for bird watchers, particularly “listers” who are keen to see every species we have on offer here in the Hauraki Gulf – especially those from overseas, who will likely only get one crack at seeing them.

Nearly two million Cook’s petrels breed on Hauturu (Little Barrier Island). There are a few breeding (or trying to breed) on Great Barrier island, and another small population is found on Whenua Hou (Codfish Island), off Stewart Island. Pycroft’s petrels, however, only breed in the Hauraki Gulf – on the Poor Knights, Hen & Chickens and the Mercury Islands, off the Coromandel Peninsula. The total world population is somewhere between 15,000 and 30,000 birds, with around 75 per cent breeding on Red Mercury island. So if you are out in the Hauraki Gulf, especially in the outer Hauturu Gulf, north of Hauturu, you’re going to see many, many more Cook’s petrels than Pycroft’s petrels. The overwhelming odds only seems to excite some birders in their quest to identify a Pycroft’s – checking every bird flying by.

How easy is it to do this? It’s very hard because, for one thing Cook’s petrels, can be very variable in their plumage. And they do look remarkably similar as the picture above of two birds sitting on the water shows; on the left is a Cook’s petrel, on the right is a Pycroft’s. In flight, identification is made all that much harder. The birds differ in their calls, but you’d have to be on the breeding islands or anchored close by to hear a Pycroft’s.

The biggest difference between the birds has nothing to do with appearance but where they migrate to once they finish breeding here in northern New Zealand. Tracking studies show Pycroft’s head to an area between the Hawaiian Islands and the Equator. By contrast, Cook’s from the Hauraki Gulf go further north, into an area between Hawaii and the Aleutian Islands and across to California.

There have been attempts to establish new colonies, the first on Cuvier Island, and Motuora in the inner Hauraki Gulf. These were done by translocating chicks, moving them to nest boxes on their new home islands and then artificially feeding them in the period before they fledge and depart. It is too early to be sure that the Motuora colony will establish, however the Cuvier Island project looks very promising. It could be that its proximity to their main breeding grounds on Red Mercury helps.

For the record, Pycroft’s are smaller than Cook’s and have different proportions – longer tail, shorter wings and darker colouration around the eye. If you want to keep yourself occupied on the next summer run to the Bay of Islands or Tutukaka, check out those dynamically flying, beautiful, Cook’s petrels. Somewhere among them will be the enigmatic Pycroft look-alike.

Chris Gaskin, Northern NZ Seabird Charitable Trust
chris@nzseabirds.com