Seasonal migration is a widespread phenomenon, which is found in many different lineages of animals. This spectacular behaviour allows animals to avoid seasonally adverse environmental conditions to exploit more favourable habitats. Migration has been intensively studied in birds, which display astonishing variation in migration strategies, thus providing a powerful system for studying the ecological and evolutionary processes that shape migratory behaviour. Despite intensive research, the genetic basis of migration remains largely unknown. Here we used state-of-the-art radio-tracking technology to characterize the migratory behaviour of a partially migratory population of European blackbirds (Turdus merula) in southern Germany. We compared gene expression of resident and migrant individuals using high-throughput transcriptomics in blood samples. Analyses of sequence variation revealed a non-significant genetic structure between blackbirds differing by their migratory phenotype. We detected only four differentially expressed genes between migrants and residents, which might be associated with hyperphagia, moulting, and enhanced DNA replication and transcription. The most pronounced changes in gene expression occurred between migratory birds depending on when, in relation to their date of departure, blood was collected. Overall, the differentially expressed genes detected in this analysis may play crucial roles in determining the decision to migrate, or in controlling the physiological processes required for the onset of migration. These results provide new insights into, and testable hypotheses for, the molecular mechanisms controlling the migratory phenotype and its underlying physiological mechanisms in blackbirds and other migratory bird species.

Blackbird_Blood_TranscriptomeMultifasta file containing the sequences of the blackbird transcriptome. The assembly was generated de novo using the program Trinity v20140717 (Grabherr et al. 2011) with the default k-mer size of 25 and a minimum contig length of 200 bp. To identify transcripts corresponding to coding genes, the obtained assembly was subjected to sequence similarity searches against a custom database containing the available bird proteins (chicken, duck, flycatcher, turkey and zebra finch; source: Ensembl release 77) and the well-annotated human and mouse protein datasets (Ensembl release 77).Blackbird_genotypesFiltered individuals’ genotypes stored in a VCF file generated by the Bayesian genetic variant detector FreeBayes v1.0.2 (Garrison & Marth), run with default parameters.