Why do Cinereous Vultures select certain sites for nesting?

Why did Cinereous Vultures choose the left tree for nesting, and not the right one?

While monitoring Cinereous Vulture nests in Portugal during the fieldwork for the LIFE Aegypius Return project, our field workers often wondered why vultures choose specific sites and trees for nesting, while ignoring others. To find the answer, Linn Lembke, a Master's student at the University of Coimbra, studied nest-site selection of Cinereous Vultures in Portugal and across Europe on two scales.

Looking at the landscape

The breeding colonies included in the study. (+ Field work for the tree-level part of the study was done in Vidigueira, Contenda, Tejo and Malcata.)

To understand the bigger picture, Linn analysed 938 cinereous vulture nests from ten breeding colonies across Europe. Besides the Portuguese colonies, VCF partners in Spain, France and Greece shared nest locations and information about the tree species used. Using geographic mapping software, the different nest sites were compared to identify common patterns and regional variances.

Surprisingly, altitude did not seem to be a limiting factor. Cinereous Vultures nested at elevations ranging from around 100 metres to more than 1,600 metres above sea level. Steep slopes, however, were selected in every region. Mountains and hills create rising air currents that make it easier for these large birds to take off and gain height. The direction a slope faces also mattered, but differently depending on the climate. Cinereous Vultures were more likely to choose north-facing slopes in warmer regions, while in colder areas they more often nested on south-facing slopes. This suggests that vultures adapt their nesting behaviour to local environmental conditions. The direction a slope faces can influence the temperature at the nest, which is important because both adult vultures and their chicks need to maintain a stable body temperature throughout the nine-month breeding season. Cinereous Vultures begin breeding in February, when temperatures in colder regions, such as the French colonies, can fall below 0°C. Nesting on warmer, south-facing slopes may therefore help adults and chicks cope with these cold conditions. At the same time, their breeding season is very long, with some chicks remaining in the nest until September. In warmer regions, this means they can experience intense summer heat. There, north-facing slopes may help reduce the effects of extreme temperatures.

The study also showed that vultures avoid human disturbance. On average, nests were located more than three kilometres from paved roads and more than four kilometres from villages.

A summary of the results found. Elements from Flaticon were used to create this figure.

Choosing the right tree

Cinereous Vulture nest in Maritime pine (Pinus pinaster)

The study also examined which tree species Cinereous Vultures use for nesting. Most nests were built on pine trees or evergreen oaks. In Portugal and Spain, Maritime Pine (Pinus pinaster) was the most frequently used species, while Scots Pine (Pinus sylvestris) was common in the French colonies. Where suitable pine trees were not available, the vultures mainly nested in Holm Oaks (Quercus rotundifolia). This was the case in the Portuguese colonies of Tejo and Douro.

Cinereous Vulture nest in Holm oak (Quercus rotundifolia). 

Additionally to tree species, other factors which could influence which tree is chosen were investigated too. Linn visited four Portuguese colonies together with the LIFE Aegypius Return field team. They measured 76 nesting trees and compared them with 103 nearby trees that were not used by vultures. The measurements included tree size and the distance to neighbouring trees.

The results showed that two factors were especially important. Cinereous Vultures preferred trees with wide canopies and further away from neighbouring trees. Bigger trees can probably support the species' large nests more safely, while open space around the tree likely makes landing and taking off easier for these birds, whose wings can reach almost three metres in length.

Helping vultures through science

The results of this study can guide conservation decisions. They help identify how habitat management can improve breeding conditions. The results can also help to identify promising nesting areas. By understanding what makes a suitable nest-site, we can better protect the habitats that Cinereous Vultures need to breed successfully.

About the project LIFE Aegypius Return

The LIFE Aegypius Return project is co-financed by the European Union's LIFE programme. Its success depends on the involvement of all the relevant stakeholders and the collaboration of the partners: the Vulture Conservation Foundation (VCF), the coordinating beneficiary, and the local partners Palombar - Conservation of Nature and Rural Heritage, Herdade da Contenda, the Portuguese Society for the Study of Birds, the League for the Protection of Nature, the Transhumance and Nature Association, the Nature and Humanity Foundation, the National Republican Guard and the National Association of Rural Owners of Game Management and Biodiversity.