The Primary Suspect ‒ The Eurasian Jay!
Assaf Ben-David, Tamar Dayan, Ido Yitzhaki
Who is preying on the nests of Sardinian warblers (Sylvia melanocephala) in the woods of Ramat Hanadiv?
This was the question put forth by the researchers* ,who set out to learn how forests of newly-established Aleppo pine trees (Pinus halepensis) are making life easier for birds of prey ‒ and more dangerous for the songbirds they threaten.
In the past decade, Aleppo pine trees began sprouting in Ramat Hanadiv's natural woodland. The growth of these tall trees changed the general height of the woodland, which led us to ask how this change impacts the predators of songbirds in general, and in particular during the nesting season. We know from other studies around the world that the nesting season is a sensitive period for songbirds, both because of damage to their eggs and nestlings, and because of the resources the birds require in order to support their offspring.
In our study, we checked how the establishment of Aleppo pines affects the predatory attempts on the nests of Sardinian warblers in Ramat Hanadiv's woodlands. Our primary suspect was the Eurasian jay (Garrulus glandarius), which European research has shown to be a very active nest raider in forests.
Our hypothesis was that, because the Aleppo pines made it easier for the jays to move around in low woodlands and observe nesting areas, the predation in the warblers' nests was greater in those places where the pine trees had settled themselves.
We carried out our research in four habitats, each with a different density of pine trees, using two methods:
1) Installing dummy eggs in nests, and identifying predators according to the marks they made on the eggs.
2) Aural monitoring of the Sardinian warblers' warning calls (translated loosely from the birds' language as 'Get out of here, you predators!') and the 'conversational' voices of the jays.
In documenting the predation, we found a wide range of predators: rodents, jays, European glass lizards (Ophisaurus apodus, also known as legless lizards), even cows that unwittingly stepped on nests. The largest number of nests that had been damaged by predators was found in a grove with scattered pine trees (75%) and in an open pine forest (60%). The lowest number was found in a maquis with no pines at all (42%) and in dense pine forests (32%). Analyzing the comprehensive statistics, we found that the jays are more active in areas with scattered pine trees.
The amount of warning calls among the warblers was highest in the open pine forest (an average of 13.6 incidents), and in the groves of scattered pine trees. The lowest was in the woods with no pine trees (an average of 4.5 incidents).
When jays' activities in the field were monitored, it was noted that they could be observed primarily in flight in areas without pine trees; in areas with pine trees, it was found that the birds were observed primarily as they were seeking and plundering nests. Despite the fact that we also identified rodents and reptiles among the predators of nests, we estimate that the highest level of predation in woods that have been invaded by pine trees and in the open forests is related to the jays. We assume that the jays use the pine trees as lookouts into the woods, and the threat of predation on songbirds' nests is therefore higher in the areas where the pine trees have established themselves.
*The researchers: Assaf Ben-David and Tamar Dayan, of the Zoology Department at the George S. Weiss Faculty of Life Sciences, Tel Aviv University; and Ido Yitzhaki, of the Department of Evolutionary and Environmental Biology, Haifa University.