As we explained in Chapter 1, environmental variables, including the weather and climate, can only influence bird populations if they alter demography: the reproductive or mortality rates and, for the subdivided parts of a closed population, immigration and emigration as well. In the previous chapter we discussed the effects of phenological mismatch on bird populations, but that is just one of the ways in which climate change can have an impact on demography (Table 4.1). These other mechanisms are the focus of this chapter. We will not restrict ourselves only to studies of climate change impacts, but also review the wider range of studies which have looked at the relationships between bird population processes and temperature, precipitation and other weather variables. Whilst many of these will really be examining the effects of annual variation in the weather (as opposed to long-term trends in climatic averages), they may still be useful in helping us understand the impact of climate change upon bird populations in the future.
Long-term studies are necessary in order to adequately describe how populations respond to annual fluctuations in the weather, and especially to see how population size is affected by longer-term changes, including recent climate change (which we regard as a long-term change in those weather variables, ideally over a minimum 30-year period, although many studies putatively demonstrating impacts of climate change span shorter periods). One of the longest such studies is that of the annual heronry census, coordinated by the BTO since 1928, which has been used to demonstrate the sensitivity of grey heron Ardea cinerea populations to cold winter weather (North 1979; Reynolds 1979). The impact of severe winters can be clearly seen leading to periodic population declines, but in response to a run of mild winters from the late 1980s to late 2000s, the population remained high and stable (Figure 4.1). Such large-scale population monitoring programmes are now widely established across Europe and North America, and often use the observations of amateur ornithologists, collected using standardised methods (e.g. Anders & Post 2006; Gregory et al. 2009; Moller & Fiedler 2010) to deliver large-scale monitoring for the production of robust population trend estimates (van Strien et al. 2001; North American Bird Conservation Initiative, US Committee 2011). However, for many species and countries elsewhere, these annual monitoring data do not exist, which is an obstacle to scientific understanding and effective conservation action (Amano & Sutherland 2013).