Ever wonder how the thousands of seabirds that visit our coasts each summer are counted? Doug Gilbert, Head of Reserves Ecology, gives us some insight.
Counting seabirds: Start at 1 and keep going
Counting seabirds – start at 1 and keep going! All very well but how do you keep track of numbers in a vast colony on a vertical sea cliff? Well there are a number of different ways depending on the colony size and habitat and most importantly the species that you are counting.
Cliff nesting birds that build obvious nests, for instance kittiwakes, gannets and shags are relatively easy. By counting the number of “Apparently Occupied Nests” or AONs in the trade, you can easily estimate the number of pairs on the cliff. In very large colonies you might only count a small plot on a regular basis to give an indicator of how numbers might be doing in the rest of the colony.
Kittiwake and shag by Andy Hay (rspb-images.com)
Guillemots don’t build any nests although they do defend a small section of cliff ledge as a territory. The best way to estimate the numbers in a consistent way which can be repeated year after year is to make several counts of all the individual guillemots present over a period in early to mid June. This is when adult attendance at the colony is at a peak. The number of individual birds present is then averaged, or a best estimate is made if the weather or other conditions make some counts less reliable. Converting this into an estimate of pairs rather than individuals is a bit less reliable than counting AONs but we can still compare numbers over time to look for declines or increases. Razorbills are counted in the same way but they are often hidden away in rock crevices on the cliff so are more difficult!
OK, but what about puffins? They lay eggs and rear their young at the end of a burrow which you can’t see in to! To make things worse, lots of puffins that are not breeding visit colonies and hang around getting their pictures taken by visitors. How do you find out whether the breeding numbers are changing? The technique is not easy but it is possible with patience and skilled observation. By using photographs of sections of the colony and marking burrows that are being used regularly by puffins carrying fish, you can estimate how many burrows are occupied by breeding pairs in any one year (Apparently Occupied Burrows or AOBs). Over several seasons it is possible to follow trends in numbers in a particular colony or area. This does take a lot of time though and is only done in a few places where counters are available.
puffin by Andy Hay (rspb-images.com)
Right then, what about storm petrels and manx shearwaters – not only do they breed in burrows but the non-incubating adults only visit at night! Monitoring the numbers of these birds is extremely tricky and the best technique we have involves playing a tape recording of the species concerned at the entrance to their burrows and recording how many birds respond to the call. We know that not all petrels in burrows respond so working out what the numbers mean is a matter of statistics based on average response rates – and get this – each colony might have a different response rate! So using a response rate from one colony might not give you a very good estimate at another. Male and female petrels also have different response rates so it gets even more complicated!
Many gulls and skuas nest on moorlands on the ground so surveying them is often a matter of watching from a vantage point and counting the numbers of territories you can see using the birds behaviour as clues.
Arctic skua chick by Andy Hay (rspb-images.com)
Counting seabirds can be smelly, exciting, cold and intriguing work but when you are back at the office and you can add a dot to a map or a point on a graph showing this years results, it all makes it worthwhile.
Counting seabirds has never been so important a task as it it is now. With many seabirds in real trouble, all that effort by hundreds of people over the years across Scotland gives us the raw data that we need to show politicians the desperate need for better management of the marine environment, including the designation of Marine Protected Areas (MPAs), and more action on climate change.