With the winter migrants returning to arctic regions to breed, the most numerous bird on the Basin is the eider. They are resident, of course, and breed around the grassy margins of the western end of the reserve.
They are one of the first birds to pair up and the males' entreating "coo" calls can be heard from December onwards as the "rough wooing" of the females takes place. These antics are easily seen from the road bridge as these ducks swim around on the low tide.
Like all birds, trying to estimate the size of the population, the breeding success and their ultimate fate is difficult and far from accurate. Out of a European population of 2 million, the UK hosts some 64,000 birds. This represents a steady improvement in numbers after considerable persecution in the whole of the UK in the 19th Century.
The other major sites in Scotland for eider breeding are the islands in the Forth estuary (around 1,000 nests), the Isle of May (with around 1,000 nests) and Horse Island (off the Ayr coast, with around 400 nests).
There has been a regular census of breeding eider on the Basin over the past six years and the data collected shows that there are about 500 breeding pairs on the Basin which are quite successful with about 50% of nests successfully producing ducklings.
This means that, as each female lays between 4 and 6 eggs per nest, about 1,250 ducklings should be hatched each year.
That is only part of the story, however, as an eider duckling is a tasty morsel for foxes, stoats, herring gulls and greater black-backed gulls. To defend against the bird predators, eider ducklings are gathered together into "crèches" of up to 100 where the "safety in numbers" principle holds and the attendant adults can defend the youngsters quite successfully. There are still some losses, however.
In addition to counting nests as a method of "observing" a population of birds, we can capture them and put identifying rings on them. The rings can then be recovered (often from dead birds) and data collected about individual birds. Recoveries of ringed eider in the Montrose area show that 44% of birds die within their first 5 years. However, 2.3% survived into their 30th year, which is a good age. The rest of the recoveries show an even distribution of ages from 5 to 25.
Nature's checks and balances mean that the population of eider on the Basin is pretty well constant from year to year. This will remain the situation unless man's activities intervene.