The value of GPS tracking in woodcock studies

Our tracking work has revealed the astounding journeys made by migrant woodcock each spring. © Andrew Hoodless/GWCT

BACKGROUND Britain and Ireland support a relatively small resident breeding population of woodcock estimated at 55,240 males in 2013, which has undergone a 56% decline in range since 1970. The woodcock was red-listed as a ‘Bird of Conservation Concern’ in December 2015 owing to the contraction in its UK breeding range. The European breeding population is estimated at seven to nine million males and shows a stable trend. In winter, we see an influx of 800,000-1.3 million migrant woodcock from Norway, Sweden, Finland, the Baltic states and Russia. Because woodcock are not easily observed, we are dependent on tagging individuals to learn about their habitat requirements, movements, breeding and survival. Tags using GPS technology are now available for smaller birds, such as the woodcock, enabling us to gain detailed insights into their movements and behaviour. Our woodcock research has long depended on tracking individuals to study their behaviour and learn their fates. Various radio-tracking studies (employing a VHF transmitter on the bird and a hand-held receiver) since the late 1970s have uncovered the breeding system of the woodcock, details of habitat preferences during the breeding season and in winter, and survival rates. More recently, our satellite tracking (using tags transmitting to Argos satellites) has revealed the astounding journeys made by migrant woodcock each spring. Now, small GPS loggers, which passively record satellite signals to estimate location, are enabling us to collect many more locations (typically 400-700) of far greater accuracy (generally to within five to eight metres) within a 12-month period.

We urgently need to understand how best to manage habitat for breeding woodcock and the factors influencing breeding success. Since summer 2015, we have been using GPS tags to study woodcock during the breeding season. We have obtained data on the habitats used for feeding and roosting during the day, and the locations of foraging sites visited at night. For instance, male woodcock in Nottinghamshire left woodland, on average, on 19% of nights to visit arable fields, rough grassland and heathland that were typically within 400 metres of the woodland boundary. There was high variation between individuals, with just over half (53%) of tagged birds feeding entirely within woodland at night (at ride sides, clearings and clearfells). This work is ongoing, to build up a larger sample of birds in a wider variety of locations and to gather data from females as well as males. The eventual aim is to use the information to create management guidelines enabling managers to improve woodland for woodcock.

Because GPS tags can record movements at very short time intervals, they are useful for examining the roding display of male woodcock. GPS tags have allowed us to determine the frequency and timing of roding, plot display flights and estimate roding areas. Previously, the average roding area, crudely estimated by two or three observers simultaneously monitoring a radio-tagged bird, was 88 hectares (ha). Our data from 16 roding males with GPS tags suggests that roding areas may be larger than this. While there is reasonable consistency in the irregular loops comprising the display area used by the same bird on consecutive nights, the tracking has shown large variation in the display patterns of individuals (see Figure 1).

Figure 1 Tracking data from two male woodcock recorded during the dusk display period. Each bird was recorded for 90 minutes per evening for two days: 12/06/2016-13/06/16 (blue) and six days: 29/04/2016-04/05/2016 (pink). These birds were caught in clearings less than 200 metres apart but demonstrated different roding behaviour. (Landsat/Copernicus image © Google Earth)

Figure 2 Migration route of a woodcock tagged in Cornwall in February 2016, showing an autumn migration route more northerly than its spring route. This bird left its winter site on 23 March 2016 and arrived at its breeding area in Russia on 9 April 2016. It left its breeding site on 3 November and was back in Cornwall on 24 November. (Landsat/Copernicus image © Google Earth)

GPS technology is also helping us fill gaps in our understanding of woodcock migration and the behaviour of migrants on their breeding grounds. By deploying geolocators and satellite tags over the last seven years, we now have good knowledge on the timing of migration, origins of migrants wintering in Britain and Ireland, and spring migration routes. However, owing to the poor solar charging of satellite tags on woodcock between late summer and late winter, and the low positional accuracy of geolocators (typically 50-150 kilometres), we are lacking high-quality data on autumn migration routes and the behaviour of birds at breeding and wintering sites. GPS loggers are much cheaper than satellite tags (about a tenth of the cost) and can record at least two accurate locations a day for a year without relying on solar charging. Despite the need to recover the logger to download the data, we have been able to obtain data from 22% of the 68 deployed over the last two years.

GPS tags have revealed that woodcock generally migrate at heights of 500-1,000 metres (1,500-3,000 feet). We have recorded some discrepancy between spring and autumn migration routes, with woodcock typically making their way to Britain and Ireland in autumn via a more northerly route than that taken to the breeding site in spring (see Figure 2). Although we plan to collect more data and conduct a formal analysis of habitat selection, movement data obtained to date suggest regular use of woodland edges and relatively young woodland on the breeding grounds (see Figure 3).

KEY FINDINGS GPS technology provides data of appropriate resolution of both space and time to enable us to investigate woodcock roding behaviour. GPS tags are yielding accurate data on habitat use at British and foreign breeding sites, which will help us devise best practice management guidelines. We are now able to complete our work on woodcock migration, gaining a better insight into nocturnal flights and autumn migration routes.

Andrew Hoodless Chris Heward

Figure 3 Daily breeding site locations (taken at 12.00 GMT) near Khiytola, Russia, between 31 May and 13 September, showing a preference by this bird for young woodland and edge habitat. This woodcock was probably a male and is the same bird as in Figure 2. (Landsat/Copernicus image © Google Earth)