Rees, R.A. & Tucker, J.J. (2020)
Descriptive models for first-arrival dates for migrants
Ringing and Migration. 34(1):1-7.
This forum article describes the analysis of first-arrival dates of migrant birds in Shropshire from the 1880s to 2010s. The process of analysis is described here in some detail, firstly to encourage similar data compilations from other areas and secondly to indicate features of data handling and analysis where special care must be directed; such manipulations are generally glossed over in write-ups!
The value of first-arrival dates has been debated, but Sparks et al (2001) concluded that trends in first-arrival dates over time constitute robust data that can be related to environmental changes. Shropshire is a landlocked county in the English Midlands, where the records are less affected by local weather events creating falls of migrant birds than would be the case in a coastal area. It is striking that we found simple trends for many species. We examined the data to look for periods of sustained change or trends. These are investigated further here with regard to climatic or biological factors (as e.g. by Sparks & Braslavská 2001), but finding different patterns between species in our data might prompt such investigations. The timing of first arrivals will however be influenced by factors all along the migration route (Gordo 2007).