Longer term trends in arrival timing of long distance migrants: the influence of abundance and population change

The first arrival dates for summer migrants in Cheshire and Wirral indicate a longer term trend towards earlier arrival since the mid 1970s, consistent with the findings of previous studies based on first arrival data from UK area bird reports. The aim of this study was to investigate the influence of abundance and population change on the apparent advancement of arrival dates. Stronger apparent advancement in arrival was observed for species which have increased in numbers in the county over recent years, for example Hobby Falco subbuteo. Less common and declining species, such as Turtle Dove Streptopelia turtur, Yellow Wagtail Motacilla flava, Wood Warbler Phylloscopus sibilatrix, Willow Warbler P. trochilus and Spotted Flycatcher Muscicapa striata, showed some of the lowest rates of advancement. These observations can be interpreted in terms of the dependence of the first arrival date on the probability of detection of individuals, which is dependent on the level of abundance of a species as a whole, and hence on changes in abundance. Theoretical considerations indicate that changes in abundance are likely to have more of an impact on the apparent advancement rate of arrival for less abundant species than for more abundant ones. Such factors may in part explain the regional differences that have been observed in apparent advancement rates for some species.