*Research Alert*
Phenological mismatch between breeding birds and their surveyors and implications for estimating population trends. Read more below #BTOScience #ornithology

Dario Massimino, @harris_sjh, @simon_gillings
@_BTO @RSPBScience @JNCC_UK

Over recent decades, various migrant bird species have started to arrive in the UK earlier each spring & some migrant & resident species now breed earlier. Has this affected the results of bird surveys, or have the people counting birds also shifted their timing? #BTOScience

Data from the Breeding Bird Survey were analysed to answer this question, which could potentially affect the population trends we produce each year. Data from 1994–98 were compared with data from 2013–17 for South-East England.

BBS volunteers significantly advanced survey dates by 2-4 days on average over this 20 year period. Median detection dates for 68 bird species had advanced by 0.82 days on average. Most changes were ±2 days and only 5 species showed a significant advancement. #BTOScience

Changes in species’ timings alone were shown to be capable of inducing apparent changes in population trends. Similar effects were found if survey timings changed. The effects varied for different species’. #BTOScience

The average effects on the population trends caused by shifts in the timing of birds & their surveyors were small, but if they become greater, they will have an increasing influence on the trends produced for some species.

To conclude, we recommend that organisers of biodiversity monitoring schemes assess these biases and that volunteers are consistent with their survey visit dates and informed of the importance of this.

Read more at: http://bit.ly/BBS_Phenology 

#BTOScience