An update from Lucy Capstick, PhD researcher at the University of Exeter, on SBS’s corvid population ecology research study.
My PhD project (funded jointly by SongBird Survival and the University of Exeter, in collaboration with the Game and Wildlife Conservation Trust), investigating how variation in the ecology and behaviour of Magpies affects their impact on songbirds, is nearing completion. The final season of field work has just finished and over autumn and winter of this year the collected data will be analysed and consolidated.
This project has examined whether individual Magpies differ in the extent to which they predate songbird nests, particularly exploring potential differences between territorial and non-territorial birds. To this end the Magpie and songbird populations of a small mixed farm in Warwickshire have been observed and monitored through spring and summer for the past three years. The continuous use of the same site, a farm with management typical of much of the UK’s rural landscape, has allowed the development of a more detailed picture of the population ecology of both the Magpies and the songbirds. It has also made it possible to investigate variation between years, for example whether differences in weather conditions influence songbird nest success or Magpie food preferences.
In 2016, as in previous years, songbird nests were located in hedgerows on the field site and monitored in order to assess Magpie impact on the songbird population. However, experience gained in the past two years meant that in 2016 a larger sample of approximately 80 nests was found, and in 50 of these, the outcome (success/failure) was known. The nests found largely belonged to more common species, almost three quarters of nests were Blackbird, Dunnock and Wren, however Goldfinch, Yellowhammer and Whitethroat nests were also located. As well as finding individual nests the territories of songbirds were also identified by mapping singing males early in the season. This information will be related to our knowledge of the Magpie population to see if, for example, the presence of a Magpie territory affects songbird territory location and nest success.
Our understanding of the Magpie population has been developed using a number of techniques over the course of the project. In 2016, Magpies were trapped across the site using Larsen traps for the third consecutive year. Trapped birds were colour ringed so they were individually identifiable. In 2015 the rings were more successfully attached than in 2014, so there was less overwinter loss between 2015 and 2016 than the previous year. Of the 75 occasions Magpies were trapped in 2016, new birds were caught only 10 times, while on the other 65 occasions previously ringed birds were re-trapped. The trapping process was slightly less successful than in 2015 and camera observations of the traps suggested there were a number of occasions of previously trapped individuals returning to traps and, perhaps unsurprisingly, avoiding tripping the mechanism again. Nonetheless, this trapping programme alongside direct observation of individuals and early season identification of magpie nest sites meant it was possible to work out where active Magpie nests were.
As well as looking at how the Magpie population as a whole may relate to songbird nest success, this project also aimed to investigate how individual Magpies differ in their impact on songbirds, for example, are some individuals more likely to predate nests in their territories than others? This year variation in individual Magpie defence behaviour was investigated, largely through work carried out by a Master’s student from Newcastle University (Rebecca Liberty). A dummy Magpie was placed near active nests and the response of the territory holder, which ranged from mobbing the dummy to ignoring it, was recorded. This variation in response, alongside other information such as differences in trappability, may indicate differences in aggression between individuals which could be linked to predation behaviour.
Another way of examining possible difference in an individual’s predation behaviour is to use artificial nests. This is a recognised technique used to gain an understanding of the pattern of nest predation and the identity of nest predators without disturbing natural nests. Both old songbird nests and manmade nests were baited with real and wax-filled quail’s eggs and monitored by trail cameras (motion activated cameras). A predator attacking a wax egg will leave an imprint, such as a beak or bite mark, and can therefore be identified. The nests were set out in transects in areas with occupied Magpie nests and in areas where it was believed Magpie presence was reduced. This method was piloted in 2015 and used more extensively this year. Over the course of the spring and summer over 400 artificial nests were placed across the field site. It will therefore be possible to look at the impact of Magpie presence on predation, but also at differences between the territories of individual Magpies, and the effect of time of year, for example, is predation higher when Magpies have young in the nest? Preliminary analysis suggests Magpie predation is higher in certain locations and trail camera pictures also identified specific ringed Magpies predating multiple nests. This may indicate that certain individuals predate more.
Over the three years of this PhD project a range of field data, including that detailed above, has been collected. The task now is to look at the information about Magpie population ecology, habitat use and territorial behaviour in combination with that about the songbird breeding population to see if and how they interact. The project will collate information about Magpie distribution, movement and breeding at a wider landscape scale. At a finer scale individual differences between Magpies in terms of territory location and habitat, and behaviour will be examined. Predation of both real songbird nests and artificial nests will be looked at with relation to the Magpie population. As well as this field data, research of the scientific literature will help to build up a more general understanding of corvid predation of songbird nests, as it could be that some species are more vulnerable to predation by corvids or that their populations are less able to recover from breeding season losses. In general this type of research into the population ecology of predators and their prey species, and how they interact, may suggest a way to manage the UK’s countryside to benefit the prey species, such as threatened songbirds.