The decline of a peripheral population of the European robin Erithacus rubecula
José Luis Tellería
This paper examines the potential effect of some factors involved in the decline of a winter population of the European robin Erithacus rubecula in southern Spain. The area receives extra-Iberian immigrants that come into contact with resident conspecifics. Given this interspersed distribution, three hypotheses were tested to explain the decline: a) the depletion of the carrying capacity of the area during winter, b) the decline of the resident population, and c) the reduction of foreign individuals in this wintering area. Winter robin and fruit (a main winter food) abundances were assessed at 43 sites from 1999 to 2014, and the abundance of breeding robins was evaluated from 1998 to 2012. Rainfall and temperatures were recorded throughout this period and changes in forests occupied by breeding robins were assessed in 1999 and 2013. Results showed a decline in winter temperatures and an increase in fruit availability. Given that food increases could buffer the thermoregulatory costs of colder conditions, the depletion of the carrying capacity of the area was discarded as a main driver of robin decline. Spring counts showed a reduction of 45% in the breeding population that was not explained by changes in forest management. However, a significant increase in summer temperatures could boost the pervasive effect of drought on robins inhabiting this marginal area of the species' range. Foreign immigrants, despite the increase in breeding populations in most of Europe, did not buffer the decline caused by the reduction of residents. In fact, an analysis of ring recoveries reported in this area depicted a sharp decrease in extra-Iberian wintering individuals. These results may be related to the effect of global warming on migratory schedules (the northwards retreat of wintering areas) and on the suitability for robins of local habitats in this peripheral region of the Palaearctic.