What is situation with Little Terns in Australia; I have read contradictory
things recently about the two races in Australia (resident breeding and
northern migrants). What are the two spp involved and what are their ranges?
I will attempt to respond to this, as no one else has done so on the list. This is as I understand it:
The Little Tern Sterna albifrons subspecies sinensis is the only Little Tern present in Australia. There are three populations in Australia.
The east coast population is the one which receives all the publicity for being under threat. Its breeding stronghold is coastal NSW, Victoria and Tas, but it breeds quite far north, to around Cairns, Qld, although in rather smaller numbers north than south. Its breeding period is during the spring and summer. A great deal of conservation work has been done on this breeding population, and where some colonies occur, local environmentalists are active in helping NPWS to guard nests from disturbance and limit predation. This population disperses after breeding, but it is not known if there are set migration routes, or whether it is just scattered dispersal. Last summer we picked up a trend of movement of this population northwards through Caloundra, Qld, in late February. We know this because of the work done in southern NSW and Victoria individually colour marking birds at the natal colonies. I understand that with some odd exceptions, Little Terns (all populations) clear completely from SE Aust after breeding.
The northern Australian population, whose breeding stronghold is the Gulf of Carpentaria and eastern Cape York, Qld. This population breeds during autumn-winter, and its dispersal behaviour, if at all, is not understood. I guess that's because of its remoteness. There needs to be some colour flagging done at the nests.
The northern hemisphere population, which breeds in places like Japan and China during the Australian winter (northern hemisphere summer), and which migrates to Australia for the Australian summer. In Aust they can be spread all along the east and south-east coast, as well as west into north Western Australia. They arrive here around Sept/October and leave in April/May. They gather in flocks of up to several thousand in some places, Caloundra being one of their many favoured locations. The Japanese laugh at our concern about the Little Tern, because it is so abundant in Japan.So far there has been no way of separating the populations. There is a slight difference in the beak tip which can be picked up in breeding plumage, but it is very slight. I have yet to master this distinguishing feature.
I have pondered much about the northern Australian population, and where they go, if they go. There is always a population present in the Gulf at any time of the year. I wonder do we ever see them on the Caloundra sandbanks? And then I ask myself what benefit could the NAust population reap from heading south at some time during the year, and I can see none. Maybe some head north from Australia. If someone would like to debate this point, I would welcome input. I reason that if there were any time when there might be a benefit for them to head south, it might be during the cyclone season, but that is close to their breeding cycle, and I would think it unlikely.
Does the northern hemisphere population join the NAust population in the Gulf? Or do they overfly the Gulf, and head down the east and west coast? Is there really a separate Gulf population, or is there just a gradual latitudinal difference in the breeding cycles along the coast? (I think the latter theory has been thrown out.)
Some years ago there was an attempt to separate the populations with DNA testing, but it was unsuccessful. I think the authors felt that there might be a possibility of success later if DNA technology improved.
On current knowledge, the only way of separating populations is on plumage, depending upon the time of year. The bottom line is that if you see a Little Tern in non-breeding plumage, it could come from any of the populations, but then again, the time of year gives indications about likelihoods. If a bird is in breeding plumage, you can, taking into account the time of year, be reasonably sure of the population.
It's a fascinating story, and the end has not yet been told. Someone has to write it.
If I have erred in any of the above, someone please correct me if you know better. I am always on the lookout for someone knowledgeable about terns (and willing to share it) lurking in the depths of the birding-aus coverts.