Steve Brooks from the Museum and collaborators from UCL, the universities of Nottingham, Bergen and Liverpool, and the RSPB have been examining reasons for the breeding success of the Slavonian grebe Podiceps auritus. The Slavonian grebe has a UK breeding population of only 29 pairs, found in NE Scotland only since 1908.  Loch Ruthven holds the largest British population in an RSPB reserve and breeding success is known to have fluctuated annually since records began in 1970.

Slavonian grebe from Audubon's Birds of America    © Natural History Museum

The research looked at whether the fluctuations are linked to the numbers of chironomids, the group of flies on which Steve is an expert.  These midges are an important food-source for the grebe chicks.

The team analysed a sediment core from the lake by slicing it into 2.5-mm sections to separate sediment on a yearly basis.  In this sediment, they looked at the remains of chironomids, diatoms (planktonic algae which show strong seasonal trends in populations) and algal pigments.   These plant data were used to deduce changes in total phosphorus in the water and to see whether there was a link between algae and the abundance of chironomids. Trends in grebe breeding success, chironomid abundance and algal populations were analysed against climate data to clarify whether climate was the key factor behind all of these fluctuations.

The study shows that grebe breeding success is linked with chironomid abundance and chironomid abundance is linked with total phosphorus. Over the past 100 years, lake productivity and chironomid abundance have both risen, increasing more rapidly from the mid-twentieth century to the present. Fluctuations in grebe breeding success from 1970 followed the same pattern as chironomid variation, with a lag of one year. 

One of the questions of interest was whether grebe breeding success was influenced by climate variability year by year.  Because the Slavonian grebe is a relative newcomer to the UK, it is not clear how vulnerable this small population is to environmental change.  However. No correlation was found between grebe productivity or chironomid abundance and climate.  The team concludes that breeding success of the grebe depends on food availability in the form of chironomids at Loch Ruthven.

Brooks, SJ et al. Population trends in the Slavonian grebe Podiceps  auritus (L.) and Chironomidae (Diptera) at a Scottish loch  Journal of  Paleolimnology April 2012, Volume 47 (4) 631-644  doi: 10.1007/s10933-012-9587-4