Importance of artificial stopover sites through avian migration flyways

Importance of artificial stopover sites through avian migration flyways: a landfill-based assessment with the White Stork Ciconia ciconia. Arizaga, J., Resano-Mayor, J., Villanúa, D., Alonso, D., Barbarin, J.M., Herrero, A., Lekuona, J.M. & Rodríguez, R. 2018. IBIS. DOI: 10.1111/ibi.12566. VIEW

The East-Atlantic flyway is one of the main bird migration routes of the world. During spring and autumn, millions of birds use this route annually, connecting their breeding sites in Europe and their wintering areas in Iberia or western Africa (Newton 2008). Iberia is, therefore, of critical importance for the conservation of all these populations, and the availability of suitable stopovers encountered along the peninsula can be crucial for the success of migration (Arizaga et al. 2011, Andueza et al. 2014).

Open landfills seem to be playing an increasing role as target artificial feeding areas for several species, not only in their breeding areas or during the winter (Tortosa et al. 2002, Duhem et al. 2008, Gilbert et al. 2016), but also during the migration period (Berthold et al. 2004). Compared to natural food sources, landfills provide abundant and highly spatio-temporal predictable food. Nowadays, they represent one of the major worldwide sources of anthropogenic food subsidies, and opportunistic species can benefit from them by improving individual fitness (e.g. survival, reproduction) and increasing their populations (Oro et al. 2013).

There are, however, some important associated risks to the use of landfills not difficult to imagine if looking the feeding conditions at those sites (Picture 1). Risks include potential contaminants, pathogens or poisoning that may have a direct negative impact on population dynamics (de la Casa-Resino et al. 2014, Amano et al. 2016). Moreover, considering the new EU policies towards the closure of open landfills in the coming years, there is a need to study how different species make use of them all year round. In particular, the use of these artificial sites as target stopover places during migration and therefore, their importance (and risks) for some migratory bird populations is still poorly known.

Picture 1 Feeding conditions in open landfills with a group of White Storks feeding there © Ricardo Rodríguez

The European White Stork Ciconia ciconia is a Holarctic species breeding from northwestern Africa through many countries of Europe to western Asia. White Storks are one of the main avian species using refuse tips in Europe (Hilgartner et al. 2014, Gilbert et al. 2016). Overall, the species has benefited from using this anthropogenic food resource, both during the breeding season (Tortosa et al. 2002) and during the winter (Gilbert et al. 2016, Catry et al. 2017). Furthermore, the use of landfills during the migration period also seems to be high (Shephard et al. 2015), although there is still an important lack of knowledge regarding the population origin of those individuals and their use of landfills along their routes of migration.

Spain supports a breeding population of c. 33 000 pairs of White Stork. In winter, the population has been estimated to reach about 30 000 individuals. Overall, both the breeding and the wintering populations doubled over the period 1994-2004 (Molina & Del Moral 2005). Excluding Iberia, the White Stork population in western Europe (France, Netherlands, Switzerland, Belgium and Germany) reaches c. 7250 pairs (Tucker & Heath 2004, Issa & Muller 2015). Many of these birds pass over Iberia en route to their wintering areas in Africa and, as more recently seen, to overwinter in Spain (Molina & Del Moral 2005, Panuccio et al. 2017).

The aim of this study was to evaluate the role of two open landfills located just before (Zaluaga) and after (Culebrete) the East-Atlantic flyway enters Iberia through the western Pyrenees as potential important stopover sites for the White Stork populations moving along this route. In particular, we aimed to (1) determine the breeding/natal region of origin of migrants passing through this route and using landfill sites to stay/refuel; and (2) evaluate the relevance of these landfill sites for the European White Stork populations entering Iberia through the East-Atlantic flyway.

From 2009 to 2016, we visited the two study sites to look for ringed White Storks and read their rings (Figure 1). Only at Culebrete, and for the months of August-September 2014-2015, a higher sampling effort (with a visit every 5 days) was carried out to estimate stopover duration. Overall, we read 967 unique colour rings of known origin (Table 1). In parallel, direct counts (Picture 2) were undertaken on a (minimum) monthly basis whenever possible.

Figure 1 Location of the two study dumps (Culebrete and Zaluaga, dark square and triangle, respectively) and sites of origin (i.e. ringing, open dots) of color-ringed White Storks

Table 1 Number of unique colour rings read at each landfill site (Culebrete, Zaluaga) with known region of origin. 43 individuals were read at both sites

Origin Culebrete Zaluaha Total
France 362 437 799 (79.1%)
Netherlands 53 41 94 (9.3%)
Germany 42 40 82 (8.1%)
Spain 11 7 18 (1.8%)
Belgium 0 15 15 (1.5%)
Switzerland 0 2 2 (0.2%)
Total 468 542 1010


Picture 2 Group of White Storks resting on one of the study sites (Culebrete) © Jesús Mari Lekuona

Most rings were read during the autumn migration period, with a peak in August (Zaluaga) and September (Culebrete; Figure 2). Overall, mean distance between ringing sites and each landfill varied seasonally and increased from summer (July-August) to winter (November-February), with the longest distances at each landfill registered in winter (Figure 3). Models supporting transients (i.e. individuals staying less than 5 days at a landfill) fitted the data better than models ignoring them.

Figure 2 Yearly distribution of the number of rings read (%) in the two study landfill sites (CULE, Culebrete; ZALU, Zaluaga); each ring has been considered only once per month

Figure 3 Mean (± SE) distance between regions of origin (i.e. ringing sites) and each landfill site (CULE, Culebrete; ZALU, Zaluaga), in relation to the month when the rings were read

The observed population size at Culebrete and Zaluaga varied between months (Figure 4). Considering a mean population size of 338 storks in August and 782 in September, and assuming a constant proportion of transients of 0.53, the estimated total population size passing through Culebrete during August-September was 4088 (95% CI 3640-4480) storks. At Zaluaga, considering a mean population size of 206 storks in August and 125 in September, and assuming the same proportion of transients, the total population size estimated during August-September was 1208 (95% CI: 1076-1324) storks.

Figure 4 Mean (± SE) daily number of White Storks found during the whole year at each landfill site (CULE, Culebrete; ZALU, Zaluaga) according to visual censuses. There were no data for Culebrete in June

Overall, the studied landfills were used by a large proportion of the White Stork population breeding in western Europe, especially during the autumn migration period. Culebrete, within the Ebro river basin, is of particular interest, with almost 4000 storks stopping-over during the autumn migration months. Considering the potential, but unclear, effects of these artificial feeding sites on aspects such as stopover decisions during migration and the health risks linked to feeding on refuse food, our study raises important conservation concerns, especially considering the new European policies towards banning all open-air landfills.



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Image credit

Featured image: Group of White Storks Ciconia ciconia in a landfill located in the study area (northern Spain)
© Ricardo Rodríguez

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