Climate Change and Avian Population Ecology in Europe

Migratory birds are having to cope with and, if possible, adapt to global climate change. Observations and data collected in Europe have elucidated the complexity of this problem. Significant changes in migratory arrival and departure dates have been demonstrated among numerous avian populations across Europe. Avian thermal range, the difference between the maximum and minimum temperatures a species can live in, is shifting in response to climate change, and birds with greater thermal range are responding better to climate change. Life history also influences how successfully an avian species will react to climate change. Birds that can better predict and respond more flexibly to environmental changes will be more successful during continued global climate change. Research must be done not just at the species level, but also at the population level, as highlighted by differences in the Dutch and English great tit (Parus major) populations.

Global climate change refers to the fact that the overall temperature increases that are occurring on earth are not uniform. The twentieth century was the warmest of the last millennium, with an average of 0.2°C above the mean temperature of the last 500 years. The most rapid warming occurred during the final 30 years of the twentieth century (Jones et al. 2001). A global meta-analysis of over 1,700 species found significant range shifts averaging 6.1 km per decade towards the poles and significant mean advancement of spring events by 2.3 days per decade (Parmesan & Yohe 2003). As parts of a food chain are unlikely to shift their phenology at the same rate, interactions between predator and prey and other parts of the food chain are also becoming increasingly asynchronous (Durant et al. 2007). Altered phenology is also affecting the annual cycle of individual species, causing mistiming between occurrence of annual stages and the optimal environmental conditions for those stages. Data from Europe are of particular interest as temperatures there increased by nearly 1°C in contrast to the global average rise of 0.76°C since 1850 (European Commission 2005). Furthermore, Europe has been the forerunner in producing important findings on the effects of climate change on avian species.

Migratory birds undergo annual cycles that include breeding, molt, and migration. Each stage ideally occurs during optimal environmental conditions and lasts for the entire duration in which those optimal conditions are available. Photoperiod is the most critical primary cue providing birds a rough timing mechanism by which to begin a stage of the annual life cycle, and allowing birds to adapt their physiology in advance of predictable environmental changes. For some species, climate change is making photoperiod an unreliable predictor of favorable environmental conditions, because the relationship between temperature-dependent resource availability and day length changes. Additionally, range shifts and expansions in response to climate change may expose birds to novel photoperiodic conditions (Coppack & Pulido 2004). Meanwhile, physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to benefit from short-term environmental variability. These secondary cues include food supply, temperature, availability of nest sites, rainfall, snow cover, and so forth, and their degree of influence is determined by the long-term predictability of the animals' environment (Wingfield 1983). For example, although full gonadal maturation is principally controlled by photoperiod, secondary non-photoperiodic cues may alter exact breeding time by fine-tuning the time of egg-laying or gonadal maturation and/or regression, which affects subsequent events in the annual cycle (Dawson 2008, Visser et al. 2004). The timing of stages of the annual cycle must be synchronized with resource availability and other environmental conditions at each site the animal visits, and variation in habitat quality during one season or part of the annual cycle may alter a birds' success during the next stage of the annual cycle (Visser & Both 2005, Lehikoinen et al. 2004, Norris & Marra 2007).

Figure 1: A flock of Barnacle Geese during autumn migration.

Image via Wikimedia Commons. Some rights reserved.

Continental population changes have been demonstrated in European birds in response to climate change. Between 1960 and 2006 first arrival date advanced significantly among species in Europe, with increased change in intermediate latitudes and among short-distance migrants (Rubolini et al. 2007). Among 100 European bird species examined for changes in spring migration timing since 1960, species that declined between 1990 and 2000 did not advance their spring migration, while those with stable or increasing populations advanced migration significantly. Conversely, population trends between 1970 and 1990 were predicted by winter range, breeding habitat and northernmost breeding latitude, but not by change in migratory timing (Moller et al. 2008). Delayed departures from breeding grounds in the fall and early returns from wintering grounds in the spring of at least 30 Central European species have also been linked to climate change (Berthold 2001). Over the past 250 years, the spring migration data of arrival to Northern Europe has advanced in correlation with weather variation. Stopover tactics and migratory routes play an important role in determining how much climate has impacted arrival dates (Lehikoinen et al. 2004). Finally, an assessment of the French breeding bird survey determined that while avian spatial distributions are shifting polewards with changing climate, the avian rate of adaptation still lags behind climate warming by approximately 182 km (Devictor et al. 2008).

Thermal range is the difference between a species' thermal maximum and thermal minimum. The thermal maximum and minimum are the mean of local spring and summer average monthly temperatures for the hottest and coldest parts of a given populations' breeding range, respectively. Many birds are shifting their geographic ranges to cooler climates. For example, northern-temperate birds are shifting their ranges (breeding and non-breeding) to higher latitudes, and models suggest that many birds will experience substantial pressures under climate change, resulting in range contraction and shifts (La Sorte & Jetz 2010). Temperate species are predicted to experience the greatest temperature increases, and climate warming has been linked to geographical range and population changes of individual species at such latitudes. An analysis of the responses of 71 avian species to the 6-month French heat wave in 2003 showed that the most significant decreases in population growth rate occurred in species with small thermal ranges in regions with the highest temperature anomalies (Jiguet et al. 2006). Accordingly, the long-term trends of 110 common breeding birds across 20 European countries showed that the sharpest population declines were among species with lower thermal maxima (Jiguet et al. 2010a). Assessment of population growth rates of terrestrial breeding birds along the latitudinal gradient of the species ranges showed that populations breeding close to the species thermal maximum had lower growth rates, while those breeding close to the species thermal minimum had higher growth rates than those in the central part of the range (Jiguet et al. 2010b). Therefore, one cannot assume a uniform response to climate change across a species range.

A bird species' life history also influences how successfully that species will respond to climate change. Birds that migrate shorter distances are showing the most significant advances in migratory timing in response to climate change (Vegvarie et al. 2010, Tryjanowski et al. 2005, Biadun et al. 2010). Having less distance between departure and arrival sites may enable these species to better predict onset of environmental conditions and seasons at their next site, and may thus explain the stronger advancement of arrival date observed among short distance migrants relative to long distance migrants (Visser et al. 2009). Birds in less seasonal habitats, and those that eat more generalized diets, are responding more successfully to climate change than are those with more limited environmental and dietary demands (Both et al. 2010, Vegvari et al. 2010). Similarly, birds that have multiple broods per year are faring better than are those with single broods (Both et al. 2010, Vegvari et al. 2010). Birds that migrate to interior versus peripheral habitats within their range have also shown more resilience in response to climate change, as interior habitats are typically more stable and less subject to outside forces than are peripheral habitats (Biadun et al. 2010). Competition for resources between species is also being altered by climate change, such as the change in nest-hole competition outcomes between pied flycatchers and great tits in Finland. As with previous examples, this demonstrates that species with highly specific needs are having greater difficulty adapting to climate change (Ahola et al. 2007). These results collectively suggest that birds that can better predict changes to their environment and those that can respond with more flexibility to environmental changes will be more successful during continued global climate change.

Figure 2: Male Great Tit (Parus major).

Courtesy of Luc Viatour via Wikimedia Commons. Some rights reserved.

Great tit (Parus major) populations of the Netherlands and the United Kingdom have shown a differential response to changes in prey availability resulting from climate change. Breeding tits feed their young winter moth caterpillars (Operophtera brumata), which are only available for about two weeks during the spring. Dates of availability of this caterpillar have changed significantly over the past half a century in response to temperature increases. Tits from Wytham Woods near Oxford responded by changing the timing of their egg-laying with variation in temperature on a year to year basis, and have advanced egg-laying dates by about two weeks (Cresswell & McCleery 2003). On the other hand, while breeding time for the Dutch tit population is also advancing each year, their response is much weaker than that of the UK population; the emergence of caterpillars is advancing three times faster than Dutch tit egg-laying dates (Visser 2005). Studies of the Dutch and English great tit populations highlight the need to assess changes not only at the species level, but also at the population level.