Interest in Gannets has been heightened this week with a report of a possible Australasian Gannet off the English east coast. In Lothian we are fortunate to host an estimated 50,000 breeding pairs of Northern Gannets on Bass Rock - for most months of the year there are a lot of Gannets passing offshore. So many that when seawatching off this coast I'd say it is normal practice to set the brain to 'ignore-all-gannets' mode.
It is possible to land on the Bass to observe and photograph the birds at close quarters, and although the trips do not come cheap, I would recommend the experience. (For trips see the Scottish Seabird Centre in North Berwick.) I was lucky enough to participate in one of these trips to Bass Rock in June 2007. The weather was cold, windy and overcast with heavy showers threatening. We were advised that landing may be impossible, and while that turned out not to be the case, the conditions for photography were far from ideal. No shots of gleaming birds against azure sky for me.
Here is my best shot from that day. It took about 50 minutes of a stealthy approach lying down in Gannet guano to get this image of two birds with North Berwick Law in the background. Even then I had to photoshop an awkward third bird's beak out of the right hand side of the picture. (Is that cheating? and should I have gone the whole hog and removed the three blurry flying birds as well?... )
The following flight shots of adults were taken on a pelagic trip off Newcastle in September 2010 organised by Martin Kitching.
Here are some younger Northern Gannets. The first is a first summer bird and the second a second summer bird.
It is interesting to see the difference in extent of white plumage between these two birds - just a few body feathers in the first year (above) and then head, body, lesser coverts of the upperwings and uppertail coverts in the second year (below). The latter can be particularly striking birds.
Finally, here are some slightly older birds that show predominantly adult type plumage but with dark secondaries and tail feathers. These birds appear to aged as 3rd year birds (using data from birds of known age) in Bryan Nelson's monograph although the illustrations show some other darker feathers in the body and coverts. However, a rare variant of 4th year birds is described in the Collins guide for birds with completely white body plumage and coverts.
It is birds in this plumage that can resemble Australasian Gannet or Cape Gannet. The first of these birds was photographed in June 2007 at the Bass Rock. I have tentatively aged this striking bird as a 4th year with its adult body feathers and coverts combined with many black secondaries using the Collins' criteria. Both wings had a classic piano-key look that allowed easy confirmation as a Northern Gannet. The second photo, taken as this bird came in to land on the rock, revealed that only the central pair of tail feathers were yet to moult.
The second bird, photographed on the pelagic off Newcastle in September 2010 looked considerably more interesting as it approached the boat. Only as it passed overhead and away could the extent of immature feathering on the upper and underwing coverts be seen. This would appear to be aged as a 4th year bird according to Collins, but a 3rd year according to Nelson's ringing data. Maybe both ages can show this plumage, however, notice that the underside is finely spotted with black, a feature that seems to be absent from the bird above and maybe consistent with an earlier age. The second picture shows a paler secondary on the right hand side, presumably this feather would show up as a nice white secondary in a shot of the upper wing.
So, other than the presence of the white secondary feather in the right wing, how would a 3rd/4th year Australasian Gannet or Cape Gannet differ from the bird above? Well, without direct experience of the species, I can only guess, but I would imagine that the smaller size of either potential vagrant may well be one of the first real clues to its identity. The Northern Gannet is larger and bulkier, being adapted to foraging in a colder climate.
In a view like the one above, a Cape Gannet would usually show an all dark tail but would certainly show a much longer black gular stripe extending from the base of the bill down the centre of the neck. The gular stripe of would be about twice the length of that shown in the bird above. The gular stripe is bare skin and its length allows the Cape Gannet to loose heat more efficiently in arid breeding colonies.
Separating this bird from an Australasian Gannet would maybe be more tricky, although I think that the gular stripe is narrower in that species compared to the bird above, although that would be a very difficult distinction to make in the field (and possibly is incorrect). On the other hand, the Australasian Gannet has a darker grey iris which may stand out in comparison with the strikingly pale iris of the Northern Gannet.
For more on the separation of Australasian and Cape Gannets see this thread on BirdForum, particularly for a nice head shot of both species together at a colony in South Africa.