Spatial convergence of meadow passerine territory distribution with mowing delay: an experiment in lowland grasslands

We experimentally tested the idea that meadow passerines could tend to match grassland areas consistently managed for enhancing their reproductive success. In the Saône valley (eastern France), mowing was postponed until after July 15 in stable parts (meadows with controlled mowing MCM) of three study sites for 10 years. Bird territories were defined each year with the Territory Mapping method and hatching chronology was described after a systematic survey of prey carrying by adults. We arbitrarily considered that breeding was at the utmost risk of failure when at least two thirds of a territory area was mown earlier than the hatching date + 20 days. In study site 1 where mowing outside MCM was completed by July 15, the proportion of passerine territories within MCM increased throughout the study period from 15 to 60%, whereas MCM was only 29.1% of the total area. Meanwhile, the rate of theoretical failure due to mowers dropped to ca. 20%. In the two other study sites where significant surface areas outside MCM were still not mown by July 15, territorial birds tended to avoid the earliest mown grasslands: the theoretical risk of failure by mowers was usually ≤ 10% in site 2 where 74% of total area on average was mown after July 15, < 20% after 2014 in site 3 with 53% of the total area mown after July 15. Passerine density increased above 10 territories/10 ha in each study site after MCM implementation.