A paper published recently by UCC found that ‘habitat, along with region and time, are important drivers of variation in hen harrier diet’. (McCarthy et al., 2021).
The aim of this study was to investigate the winter diet of hen harrier and study the temporal and spatial variations in diet composition. The team collected a total of 1117 hen harrier pellets from 11 winter roost locations between 2017 and 2021 in Ireland. A pellet, in ornithology, is a mass of undigested fragments of a bird’s food that some bird species occasionally regurgitate. The contents of a bird’s pellet depend on its diet, but can include the exoskeletons of insects, indigestible plant matter, bones, fur, feathers, bills, claws, and teeth.
During winter months hen harrier will gather at night, in what are known as roosts. These roosts offer a safe haven for the birds where they can shelter and stay warm during the long winter nights. These roosts can be solitary or communal and the number of individuals occupying a roost can vary greatly in response to many factors such as availability of prey or disturbance. During this study the team analysed 1117 Hen harrier pellets: 242 came from four lowland coastal roosts, 268 from four lowland inland roosts and 607 from three upland roosts.
Results
The results of this study describe the winter diet of the Hen harrier in Ireland and the variation in diet composition across habitats, regions and time. Hen harrier winter diet was mostly comprised of avian prey (95.9% of pellets), with mammalian prey found in only 12.0% of pellets. Prey groups differed across the various habitats – from lowland coastal roosts through lowland inland and upland roosts. Arable areas usually found around lowland coastal roosts, paired with wild bird cover and low-intensity farming, were linked with a prevalence of small birds and small mammals in hen harrier diet. In contrast, bogs and young conifer plantation forests, which are more commonly associated with upland areas and are where the majority of large-scale commercial forestry is found in Ireland, were linked with a prevalence of medium-sized birds in the hen harrier diet (see Figure 1 below). The number of medium-sized birds in the diet varied across months, with numbers of small birds and small mammals remaining steady. There were also differences between winters in the amount of small- and medium-sized birds in the diet
Interestingly, the results of this study also show the role that non-native small mammals can have in the composition of Hen harrier diet. This study also provides the first evidence of Greater White-toothed Shrew being predated by Hen harrier in Ireland. This helps to demonstrate how adaptable hen harrier can be in relation to variation in the availability of different prey species. The frequency of Greater White-toothed Shrew in the diet of one individual Hen harrier from a lowland inland roost, shows the potential for significant contributions of non-native prey species to the diet of Hen harriers in Ireland. The author of this paper notes that Greater White-toothed Shrews are highly vocal (Siemers et al. 2009), and so, the ease with which they can be detected may help to explain their frequency of occurrence in the pellets of this individual Hen harrier. It is possible that the Greater White-toothed Shrew will become an increasingly important prey species as it spreads throughout Ireland. The Greater White-toothed shrew (Crocidura russula) is a relatively recent arrival in Ireland. Its presence in Ireland was first discovered in 2007 from the pellets of birds of prey (Tosh et. al 2008).
Conservation implications
In conclusion the author notes that there is an opportunity for enhancing hen harrier habitat through land management. The findings of this study may also be used to inform effective conservation strategies for wintering hen harrier at a landscape scale. The report mentions actions such as planting wild bird cover and leaving arable fields in stubble over winter in important hen harrier wintering areas could be encouraged in order to provide steady sources of small bird and small mammal prey. In addition, the protection and retention of wet, open habitats in lowland inland and upland areas would maintain suitable habitat for species such as common snipe, which was found to be an important component of hen harrier diet in lowland inland and upland areas during this study.
References
McCarthy, A., Smiddy, P., Nagle, T., Mee, A., Irwin, S., Caravaggi, A., O’Halloran, J., 2021. Landscape and temporal influences on the winter diet of a threatened diurnal raptor, the Hen harrier Circus cyaneus, Bird Study, 68:3, 408-421, DOI: 10.1080/00063657.2022.2103515
Siemers, B.M., Schauermann, G., Turni, H. & von Merten, S. 2009. Why do shrews twitter? Communication or simple echo-based orientation. Biol. Lett. 5: 593–596.
Tosh, D.G., Lusby, J., Montgomery, W.I. & O’Halloran, J. 2008. First record of Greater White-toothed shrew Crocidura russula in Ireland. Mammal Rev. 38: 321–326.