Publication Review: "Patterns of bird migration phenology in South Africa suggest northern hemisphere climate as the most consistent driver of change".

By Lungi Khuzwayo, with contributions from Associate Professor Res Altwegg and Alicia Okeyo.

ACDI Research Chair Assoc. Prof Res Altwegg holds an associate professorship at University of Cape Town’s (UCT) Department of Statistical Sciences is currently heading up the new Centre for Statistics in Ecology, Environment and Conservation (SEEC) at UCT, which he established in 2014. Altwegg was recently involved in a study that examined changes in the timing of bird migration. Together with co-authors Ms Elsa M. S. Bussiere and Prof Les G. Underhill from the University of Cape Town’s Animal Demography Unit, he published a paper entitled “Patterns of bird migration phenology in South Africa suggest northern hemisphere climate as the most consistent driver of change”, in the Global Change Biology. In this study, bird atlas data was used to quantify change in migration phenology for a number of species that breed in Europe and come to South Africa during our summer months. The study also looked at intra-African migrants, and long distance migrants – the species that commute between Europe and South Africa between seasons – and changes in their phenology. The study postulates that long-distant migrant birds appear to be reacting to the earlier springs in the northern hemisphere, and aimed to research these changes as means to this paper to fill a gap that was identified in a previously published, extensive literature review on phenology changes in the southern hemisphere that Altwegg co-authored with an international consortium of climate-change researchers (titled “Phenological Changes in the Southern Hemisphere”, published in the journal PLoS ONE).

In a nutshell: A review of the publication.

The paper begins by outlining the seasonal migration of animals and the different ways in which they adapt to the environmental fluctuations. The timing of migration events is shown to be an important indicator of how ecosystems react to changes in seasonality such as climate change. Extensive research has also shown that shifts in migration phenology of Palearctic bird species are occurring at their breeding grounds. However, Altwegg et al., (2012) argues that very little is known about the phenological shifts of these bird species outside their nonbreeding grounds, nor on how phenology may react to environmental changes in the southern hemisphere (Chambers et al., 2013). Since climate change patterns are not uniform across the globe, phenological responses to a changing climate are expected to show much disparity and to vary across species, depending on their degree to which they can adapt to particularly new habitats and environments.

The aim of the study was to examine the possible changes in phenology of 16 migratory bird species that were considered good representatives of the following groups:

• Palearctic terrestrial migrants (containing five study species)
• Palearctic migratory water-birds (containing four study species)
• Intra-African migrants (containing seven study species).

The selection criteria for each species were based on the abundance and robustness of available species-specific atlas data. The bird atlas data, obtained from the Southern African Bird Atlas Projects, were used to quantify the arrival and departure patterns of the migrants in the central Highveld area of South Africa at two time periods i.e. 1987-1991 and 2007-2012.

Due to the Highveld’s characteristic high latitudes, strong seasonality resulting in clear shifts towards earlier springs, and extended conditions favourable for breeding in birds, the authors expected the Palearctic migrants to shorten their stay at nonbreeding grounds in South Africa by leaving earlier and possibly arriving later after their breeding cycles. Despite the uncertainty of the influence of climate change on seasonality in African areas, Intra-African migrants were expected to have inconsistent patterns if they reacted to local changes at their nonbreeding grounds, and consistent patterns if migration phenology is driven by conditions at their South African breeding grounds. As a final point, consistent patterns were expected across all species if environmental conditions in South Africa had a strong effect on phenology.

The authors used data that were collected by the two Southern African Bird Atlas Projects for this study. Since individuals of a migratory bird species usually do not arrive all across South Africa simultaneously, the authors therefore selected a large enough area to obtain a substantial dataset. The phenological shifts for each species were analysed separately, using the bird atlas data and statistical methods suggested by Altwegg et al. (2012).

The results of the study, divided into three parts below, aimed to shed light on the observable migratory and related phonological changes of Palearctic and intra-African bird species:  

• Asymmetric phenology systems

All studied species (i.e. both Palearctic and intra-African migrants) showed an asymmetric phenological pattern throughout year, with reported rates for most species reaching a distinct plateau in the summer season, when the species is most abundant. The intra-African migrants did, however, show an asymmetric phenological pattern that was opposite to the one shown by all the Palearctic migrants.

• Changes in maximum reporting rate

The study found a change in the maximum reporting rate between the two time periods, for 12 out of the 16 species, the maximum reporting rates were higher in 2007–2012 than in 1987–1991. The greatest change was observed in the striped swallow; its maximum reporting rate increased between the two atlas periods nearly doubling it. For some species, the maximum reporting rate decreased, e.g. for three of the Palearctic migratory birds i.e. white-winged tern, common sandpiper, and wood sandpiper.

• Changes in temporal migration patterns

The study showed that changes in arrival and departure times tended to be negatively correlated across species, i.e. species that delayed their arrival tended to advance their departure and vice versa.

By examination of shifts in migration phenology in a suite of species in the central Highveld of South Africa, the study  showed that nearly a third of the species that were considered have changed their arrival dates, departure dates or both. Intra--African migrants had no clear patterns of change in their migration phenology. While, a clear pattern emerged among the Palearctic migrants that advanced departure from the study area between the periods 2007–2012 and 1987–1991. The migratory waterbirds were the main driver of the advancement of departure. Seasonal shifts in April temperature have been fastest in the high Arctic, where most of the waterbirds from the sample breed. The results from the study are therefore consistent with the hypothesis that climate change in the northern hemisphere breeding ranges is driving shifts in migration phenology of long-distance migrants, and with the hypothesis that migrants mainly time their departure in response to local conditions in South Africa.

In conclusion, the authors suggest that the changes in the northern hemisphere conditions are a main driver for migration phenology in the bird community of South Africa. Additionally, the local conditions in South Africa or the tropical nonbreeding locations of the intra-African migrants did not appear to leave consistent traces in the data. When comparing this study to a recent review by Chambers et al., (2013) entitled “Phenological Changes in the Southern Hemisphere”, their study found changes to be of similar magnitude to those observed in the northern hemisphere. Therefore, the results from the two studies are in contrast. It is noteworthy that Chambers et al., (2013) highlighted the lack of studies from the tropical regions and Africa. Hence, this review is very important as it fills a critical knowledge gap.

References

Altwegg R, Broms K, Erni B, Barnard P, Midgley GF, Underhill LG (2012) Novel methods reveal shifts in migration phenology of barn swallows in South Africa. Proceedings Biological Sciences/The Royal Society, 279, 1485–1490.

Chambers LE, Altwegg R, Barbraud C et al. (2013) Phenological changes in the Southern Hemisphere. PLoS ONE, 8, e75514.