Is it a Red-billed or Black-billed Gull?

Recently I was down at the Rotorua lake photographing and I got chatting to a confused overseas visitor who told me that he had reported Red-billed Gulls on eBird and had been “corrected” that almost all the gulls on inland New Zealand lakes would be Black-billed. I was able to reassure him that almost all the gulls around us were Red-billed, in fact in the time we were talking I couldn’t find a single Black-billed to show him.

Differentiating the 2 species can be challenging because bill colour is not a reliable identifier with both showing some black and red at times of the year and stages of development. In full adult breeding condition it is good but otherwise can be confusing. Equally leg colour and plumage subtleties can be difficult. What I find most useful is bill shape with Black-billed having a longer and finer bill than Red-billed. I find this stands out to me even when tracking a bird in flight.

Young Red-billed gull/Tarāpunga showing relatively deep bill

Compare the bill of the above bird with this next one. Both have bills with shades or red and black but the shapes are quite distinct.

Black-billed Gull/Tarāpuka showing longer finer bill

A recent trip to the lake had both species involved in a bit of a feeding frenzy, plunging for small fish. This gave repeated opportunities to track birds in flight and diving. Most were Red-billed but the Black-billed leapt out at me as obviously different due to their bill shape.