The first national survey of sand martins in Estonia was organized by the Estonian Ornithological Society in the summer of 2017. The study aimed to estimate the size of the breeding population, map their distribution and sample the breeding success in different types of colonies. In 2018 the project will introduce for the first time in the country management measures for sand martins in mineral extraction sites.
By Liis Keerberg and Riho Marja (Estonian Ornithological Society)
During the national assessment of sand martin via web application www.eoy.ee/riparia we received data about 165 observation spots by 45 participating birdwatchers. In addition to active colonies (n=93, Fig. 1) abandoned sites and potentially suitable habitats were controlled for presence of sand martins.
Based on the data of the national assessment the population size of sand martin in Estonia could be estimated to 6700-9400 pairs. The results are in agreement with former estimates (5,000 – 10,000 pairs (Elts et al. 2013) but more precise.
In 40.9 % of the colonies we detected high risk of underestimation of the actual number of breeding pairs (on average 20,5 % occupied holes per colony) due to difficulties of precise observation. To avoid biased results we excluded from further analyses such colonies and only used data with close to the average percent of occupied burrows per colony (ca 60 %) (n=55). To come up with a national population size estimate we used both groups, but corrected the occupancy rate to the average level of occupancy (from 20 % to 60 %).
The average numbers of breeding pairs is 36,4 pairs per colony, which is similar to most other European countries (Cramp 1988). More than 100 sand martin pairs were counted in 7 colonies. Those large colonies hosted 64 % of all breeding pairs. The largest colonies with 500 and 470 breeding pairs are located in sandpits of Southern Estonia. 75,3% of the sand martin breeding sites were observed in man-made habitats (mostly sand- and gravel pits) and 24,7% in natural habitats (lake and river banks). The rate of occupancy of the nest was also statistically higher in man-made habitats.
Thus in the sandstone outcrop at lake Peipus only 112 occupied nests were counted. The colony is situated at the trans-boundary lake on the border between Estonia and Russia and it has been historically one of the largest colonies in the country (2709 pairs in 1938, 2000 pairs in 2003, 678 pairs in 2012).
We admit that this distribution between natural and man-made sites might not be fully accurate as it might reflect observer bias. Access and observation at man-made habitats was much easier. Nevertheless, the results confirm the very important role of mineral extraction sites as breeding habitats for the sand martin in our country, similar to that in Western Europe. These findings support the relevance of our collaboration with Kunda Nordic Tsement to develop measures and adapt the mining activities to the needs of the sand martin.
Nest predation was registred in 27 % of all breeding colonies. Predators dig into chambers either in front or from the top of the nesting wall or river bank. The red fox (Vulpes vulpes) was identified as the most common sand martin predator was identified in 68 % of the cases. Other potential predators near to the colonies (but not in action) noticed by the observers were grass snake (Natrix natrix), Eurasian sparrowhawk (Accipiter nisus), common kestrel (Falco tinnunculus), raven (Corvus corax) and domestic cat (Felis catus).