Ornithology: Danish dairy farmer delivers data coup

Neither the name Peder V. Thellesen nor the Danish Ornithological Society Journal will resonate with most Nature readers. In a striking example of citizen science, the Danish journal has just published 45 years of Thellesen's breeding data from his studies of starlings (Sturnus vulgaris) in 27 nesting boxes on his dairy farm (P. V. Thellesen Dansk Ornitol. Foren. Tidsskr. 111, 87–95; 2017). As far as we know, this data set provides a world-class example of the effects of climate change on the natural world.

The starlings advanced the date of their egg-laying by 1 day every 5.0 years for the first clutch and every 4.7 years for the second clutch. Thellesen found that this change in breeding onset significantly correlated with the mean rise in local April temperatures over those periods. Clutch size and hatch rate remained constant, although nesting-box occupancy has fallen by 40% since 2004, in line with the bird's decline nationally and regionally.

Thellesen ringed a total of 12,450 starlings, or 1 in 16 of all starlings ever ringed in Denmark. Although he has no formal scientific training, his patient and systematic observations far exceeded the duration of any funded research project. As the language gap between scientists and the public widens, we find this work an inspiring reminder of the might of human curiosity.

Author information


  1. Aarhus University, Denmark.

    • Tony Fox
    •  & Henning Heldbjerg


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Corresponding author

Correspondence to Tony Fox.


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