The trends shown by this new research are large scale continental patterns but what factors may be driving these at a finer, individual level scale? Work done by researchers like Susie Cunningham, Rowman Martin and my pied babbler colleague Lizzie Willey is shedding light on this. Work by these researchers, and others, has shown that when temperature increases it can have dramatic effects on the reproduction of birds, their ability to forage and the viability of their social groups. Nestling fiscal shrikes grow slower when temperatures increase over a threshold level: this increases the risk of nest failure and results in smaller fledglings who are likely to incur a fitness cost. Similarly, on warmer days pied babblers are less efficient at foraging and put on less weight while foraging. If multiple extremely hot days occur in a row this could have a dramatic impact on an individual’s weight, and if they are caring for young then temperature will impact adults, offspring or both. Anecdotal evidence, hopefully soon to be verified statistically, of pied babbler groups is that in the hottest years more groups fail to raise any offspring and become extinct, presumably from the direct effects mentioned above. All of these small individual level mechanisms may help to understand the drivers behind higher level continent wide patters.
But climate change doesn’t just act on its own, it occurs in conjunction with direct habitat change caused by humans. Pied crows in South Africa have changed the areas in which they live based on a change in climate but it is facilitated by power lines. These birds need to nest in trees and the existence of power lines has allowed them to colonise treeless areas and take advantage of them.
But what does all of this mean? By understanding the lower level effects it may be possible to design or implement habitat measures that can mitigate some of these effects. This may be farfetched, as these are large scale problems. For example when putting up nest boxes for endangered species it is important to ensure that they are not going ‘cook’ the nestlings inside, it’s worth spending that extra time researching this before spending large sums of money on a conservation effort that is doomed to fail. Small and simple considerations like that may have an impact.
For more information on this type of research you can visit the Hot Birds Research Project.
Some papers that look at the direct effects:
Cunningham et al. (2013) Temperatures in Excess of Critical Thresholds Threaten Nestling Growth and Survival in A Rapidly-Warming Arid Savanna: A Study of Common Fiscals. PLoS ONE
Frequency, duration, and intensity of hot-weather events are all predicted to increase with climate warming. Despite this, mechanisms by which temperature increases affect individual fitness and drive population-level changes are poorly understood. We investigated the link between daily maximum air temperature (tmax) and breeding success of Kalahari common fiscals (Lanius collaris) in terms of the daily effect on nestling body-mass gain, and the cumulative effect on size and age of fledglings. High tmax reduced mass gain of younger, but not older nestlings and average nestling-period tmax did not affect fledgling size. Instead, the frequency with which tmax exceeded critical thresholds (tcrits) significantly reduced fledging body mass (tcrit = 33°C) and tarsus length (tcrit = 37°C), as well as delaying fledging (tcrit = 35°C). Nest failure risk was 4.2% per day therefore delays reduced fledging probability. Smaller size at fledging often correlates with reduced lifetime fitness and might also underlie documented adult body-size reductions in desert birds in relation to climate warming. Temperature thresholds above which organisms incur fitness costs are probably common, as physiological responses to temperature are non-linear. Understanding the shape of the relationship between temperature and fitness has implications for our ability to predict species’ responses to climate change.
Cunningham et al. (2013) Identifying Biologically Meaningful Hot-Weather Events Using Threshold Temperatures That Affect Life-History. PLoS ONE
Increases in the frequency, duration and intensity of heat waves are frequently evoked in climate change predictions. However, there is no universal definition of a heat wave. Recent, intense hot weather events have caused mass mortalities of birds, bats and even humans, making the definition and prediction of heat wave events that have the potential to impact populations of different species an urgent priority. One possible technique for defining biologically meaningful heat waves is to use threshold temperatures (Tthresh) above which known fitness costs are incurred by species of interest. We set out to test the utility of this technique using Tthresh values that, when exceeded, affect aspects of the fitness of two focal southern African bird species: the southern pied babbler Turdiodes bicolor (Tthresh = 35.5°C) and the common fiscal Lanius collaris (Tthresh = 33°C). We used these Tthresh values to analyse trends in the frequency, duration and intensity of heat waves of magnitude relevant to the focal species, as well as the annual number of hot days (maximum air temperature > Tthresh), in north-western South Africa between 1961 and 2010. Using this technique, we were able to show that, while all heat wave indices increased during the study period, most rapid increases for both species were in the annual number of hot days and in the maximum intensity (and therefore intensity variance) of biologically meaningful heat waves. Importantly, we also showed that warming trends were not uniform across the study area and that geographical patterns in warming allowed both areas of high risk and potential climate refugia to be identified. We discuss the implications of the trends we found for our focal species, and the utility of the Tthresh technique as a conservation tool.
Cunninham et al. (2016) Electric crows: powerlines, climate change and the emergence of a native invader. Diversity and Distribution
Climate change and other anthropogenic global change drivers act in complex, mutually exacerbating ways to alter the abundance and distribution of species. In South Africa, pied crowsCorvus albus have increased in numbers and range in recent decades. Popular opinion links these changes to urbanisation and infrastructure development, but there has been no empirical test of this idea. We aimed to clarify the drivers of pied crow population changes in South Africa.
We used publicly available long-term datasets, the Southern African Bird Atlas Project and University of Delaware Gridded Climate Database, and spatial data from government bodies, to assess relationships between pied crow population and range changes, land use, infrastructure, urbanisation and climate change.
Pied crow numbers have increased significantly in the past three decades, but rate of increase varied geographically, with crows declining in the northeast and increasing in the south-west of South Africa. Pied crow population changes were strongly correlated with climate change. Crows have benefited most from climate warming in the shrubland biomes of south-western South Africa. Pied crows are tree nesters, and within these shrublands, there is a strong positive relationship between the rate of population increase and the density of powerline infrastructure, which may facilitate pied crows’ increase by providing nesting sites.
Pied crow numbers have increased in response to climate warming, with their spread facilitated by electrical infrastructure in south-western South Africa, providing a clear example of compound influence of multiple global change drivers promoting a significant change in species range and reporting rate. Pied crows are generalist predators and there is popular concern about their ecological impact in areas where increases have occurred. We highlight the importance of understanding the ecosystem-level implications of increased numbers of pied crows in South Africa's shrubland biomes.
Du Plessis et al. (2012) The costs of keeping cool in a warming world: implications of high temperatures for foraging, thermoregulation and body condition of an arid-zone bird. Global Change Biology
Recent mass mortalities of bats, birds and even humans highlight the substantial threats that rising global temperatures pose for endotherms. Although less dramatic, sublethal fitness costs of high temperatures may be considerable and result in changing population demographics. Endothermic animals exposed to high environmental temperatures can adjust their behaviour (e.g. reducing activity) or physiology (e.g. elevating rates of evaporative water loss) to maintain body temperatures within tolerable limits. The fitness consequences of these adjustments, in terms of the ability to balance water and energy budgets and therefore maintain body condition, are poorly known. We investigated the effects of daily maximum temperature on foraging and thermoregulatory behaviour as well as maintenance of body condition in a wild, habituated population of Southern Pied Babblers Turdoides bicolor. These birds inhabit a hot, arid area of southern Africa where they commonly experience environmental temperatures exceeding optimal body temperatures. Repeated measurements of individual behaviour and body mass were taken across days varying in maximum air temperature. Contrary to expectations, foraging effort was unaffected by daily maximum temperature. Foraging efficiency, however, was lower on hotter days and this was reflected in a drop in body mass on hotter days. When maximum air temperatures exceeded 35.5 °C, individuals no longer gained sufficient weight to counter typical overnight weight loss. This reduction in foraging efficiency is likely driven, in part, by a trade-off with the need to engage in heat-dissipation behaviours. When we controlled for temperature, individuals that actively dissipated heat while continuing to forage experienced a dramatic decrease in their foraging efficiency. This study demonstrates the value of investigations of temperature-dependent behaviour in the context of impacts on body condition, and suggests that increasingly high temperatures will have negative implications for the fitness of these arid-zone birds.