Local Black Tern (Chlidonias Niger) population trends in relation to nest platform provisioning

The population of Black Terns in The Netherlands strongly declined in the period from 1950 to 1980. In recent years numbers have been more stable. The supply of artificial nest platforms has successfully attracted breeding Black Terns, and 80% now breed on platforms. In this paper we describe the development in numbers and breeding success of a Black Tern population in The Netherlands over the period 1982-2015, where ample nest platforms have been supplied over time. Using this database we question whether we can increase the breeding population by supplying platforms. We are uncertain whether tern numbers responded to an increase in nest platforms, or whether researchers responded to an increase in tern numbers by putting out more nest platforms. We analyse (1) settlement and breeding success in relation to nesting substrate, (2) the covariation between breeding numbers and platform numbers over time and space, and (3) the density dependence of settlement and breeding success. Against this background we analyse whether the annual change in platform numbers is followed by a change in tern numbers, as would be expected if there was a causal link between the two. We show that in our study area reproduction on artificial platforms was higher than on natural substrate and that the tern population in the whole study area increased over the study period. This increase in numbers was associated with an increase in the number of platforms, as expected when platforms affect tern numbers. Settlement in the first year after introduction of the platforms was higher, but in later years there was no correlation between annual change in the number of platforms and annual change in the number of breeding pairs. We did not detect a decline in the number of fledglings produced per pair with increasing tern breeding density. In conclusion, although the general pattern suggests positive effects of platform numbers on tern numbers, we are strictly speaking not able to show that supplying nest platforms caused the increase in the local Black Tern population over the study period. The answer awaits large scale experiments with control areas and individually marked birds to quantify survival and dispersal