Delta-sites or change in number of sites occupied by each study species over time (1990–2011).

Delta-sites are shown for both A) breeding and B) wintering. Delta-sites is defined as the subtraction of the average of the number of occupied sites in the second half of the time series (2001–2011) from the average number of occupied sites during the first half of the time series (1990–2000). Positive values of the index indicate species expansion. The dotted line is the arbitrary minimum value beyond which we consider range expansion is taking place.