Although the whole colony has probably been increasing in size, it is more instructive to consider the species individually.

Northern Fulmars used to be uncommon in Europe in the 19th Century and St Kilda was the earliest known site. Since then, numbers have been increasing everywhere but St Kilda is still the largest colony.

Still the largest single Northern Gannet colony in the world, numbers grew steadily up till 1994, since when there has been little subsequent growth.

It is only recently that techniques have been developed to estimate the population size of the Leach’s Petrel that nest in burrows and only come ashore at night. 45,433 occupied sites were counted in 1999 but when the survey was repeated on Dun in 2003 it was estimated that they had declined by 45%. This alarming decline is believed to be partially attributable to predation by Great Skuas, whose numbers are increasing rapidly. A re-survey of Dun in 2006 suggested that no further declines had occurred.

Unlike most species, the population of Black-legged Kittiwakes peaked in 1969 and has been falling ever since. The graph below shows that numbers continued to decline till 1999. More recent counts in 2006 shows this decline has continued with only 3032 breeding birds being counted, a 61% decline on the 1999 fugure. It is likely that the numbers counted reflect the abundance of their main prey, the sandeel.

Common Guillemots only lay one egg a year but their numbers were steadily increasing, particularly in their main colonies on Hirta, up till 1999. It is likely that this is related to food supply as, unlike kittiwakes which feed near the surface, they dive to catch fish swimming near the seabed. Counts conducted in 2005, however, suggests that this may be changing as significant declines were observed in most study plots.

Razorbills are more scattered than Common Guillemots and therefore more difficult to count. Their numbers appeared to peak at 5,111 in 1987 and have since fallen to 3,378.

Great Skuas are relatively recent arrivals on St Kilda, having been first recorded in 1963. Since then they have increased rapidly and now number 480 individuals, mostly on the grassy slopes of Hirta with a smaller colony on Soay. They prey extensively on the smaller petrels and their impact on the Leach’s Petrel colony has recently been causing considerable concern.

Monitoring and research work
St Kilda is one of the sites that is regularly monitored for seabird numbers and productivity. The NTS Seabird and Marine Ranger, Ranger and staff from the Joint Nature Conservation Committee carry out this work at a number of sample sites around the archipelago.
At longer intervals, approximately once every fifteen years, the whole colony is censused. The last full census was Seabird 2000, carried out between 1999 and 2001.

In 2001, staff from JNCC developed a new method for estimating the total number of burrow-nesting petrels, listening for the response to tape recordings from adults incubating their eggs underground. This method has been used to provide new population estimates for St Kilda petrels.

Research workers from Glasgow University, led by Robert Furness, have recently been studying the diet of Great Skuas and have concluded that they consume large numbers of storm petrels, possibly taking as many as 14,000 a year. Further research is planned to clarify this and identify whether it is the cause of the dramatic decline in petrel numbers.