"Seagulls" - the biggest inaccuracy in the Big Garden Birdwatch?

I have always felt that the position of Common Gull in the tables produced from data submitted for the BGBW is inaccurate, and have generally considered this to be due to misidentifications (largely due to inexperienced observers assuming that the "seagulls" in their garden are going to be the most common species, and that this must be the 'Common' Gull.

In the 2012 results tables, Common Gull appears at number 20 in the national table, with Black-headed Gull at number 32, and Herring Gull at number 37 - apparently with Common Gulls far exceeding the other two species combined both in terms of the number of gardens visited and in actual numbers.

The reality is that Black-headed Gull tend to far out number all other gull species inland (with Common Gulls the second most numerous species), and both Black-headed Gull and Herring Gull are generally at least as numerous as Common Gulls on the coast, if not more common.

The results for the BTOs Garden Birdwatch tends to have Black-headed Gull placed at around the 32-33 position (with around 5-6% of gardens reporting the species), while other gull species don't usually feature in the top 40 (the following are the results so far from the 1st quarter of 2013 - the "Avg Rep Rate" x 100 gives the percentage of gardens). NOTE - the BTO table is sorted by the number of gardens reporting the species, not by the numbers reported in each garden like the RSPB table:

So why does Common Gull feature so high in the tables from the RSPB results?

I would suggest that the BGBW is reinforcing the idea that a species named the 'Common' Gull must be the most common species of "seagull", and as a result the vast majority of observers who are uncertain of the identification of gulls visiting their gardens are recording them as "Common Gulls".

The "other species" section of the recording form specifically points observers towards Common Gull if they have recorded "seagulls":

It is possible for those taking part to click on the icon next to the species name to view a picture, but even if they try this it might not help because gulls will tend to snatch food and then leave immediately in all but the largest of gardens - so views are likely to be relatively brief.

I would suggest that the best option would be to indicate that Black-headed Gull was the most common species, perhaps also suggesting that Herring Gull and Common Gull should be checked.

Of course this won't affect the data for species other than gulls, so isn't of any major importance for the smaller species that are more typically thought of as 'garden birds'.