The first crop of 2023 young Curlew have started hatching this week at Pensthorpe, but what about the fortunes of birds released in previous years? Sam Franks at BTO has been leading the post-release monitoring for the project and provides an update.
In 2021, 79 young Curlew were released in Norfolk along the shores of the Wash in July and August, with another 37 following in 2022. Yellow leg flags with unique 2-letter codes, paired with an orange ring below and a yellow ring on the opposite leg denote these birds as belonging to the Norfolk headstarting project. Sightings from project staff and members of the public have allowed us to gain some insight as to where these birds have gone and what they are doing.
As of May 2023, 37 of the 116 birds released so far have been seen at least once. Of those sightings, most have been around the Wash, particularly in winter when the young Curlew have joined the flocks of wild adults that winter here. However some have been seen further afield, including the Exe Estuary in Devon (‘4P’ released in 2021) and Guernsey in the Channel Islands (‘8H’ released in 2022).
While all birds released for the project are colour-marked, and resightings of flagged birds have provided information on a relatively large proportion of the Norfolk headstarts, a smaller number of birds are wearing special technology that provides daily, in-depth information on their every movement. GPS tags have allowed us to follow the movements of birds in great detail, from the moment they are released, through their migration and the wintering period, and occasionally into their second year of life. These tags are worn like a small rucksack on the back, log locations every 15 minutes to 4 hours depending on battery charge, and then transmit data remotely over the mobile phone network.
While the data provided by the GPS tags provide us with unique and essential insight as to how the movements of headstarted Curlew compare to wild birds, not all birds are equipped with a tag. Just as you or I would be somewhat hindered if we were wearing a moderately heavy rucksack compared to someone without, so might be a Curlew wearing a tag. By only equipping a small proportion of birds with tags, we can learn essential information which can only be gleaned from tagging, while also assessing any impact the tags have by comparing the survival of tagged vs colour-marked only birds. In 2022, GPS tags provided the first glimpse into the migration routes of British headstarted Curlew: first Nemo (‘6Y’) made a perilous trip to southern Ireland which included a nail-biting diversion over the Celtic Sea; followed by Astra (‘9L’), who headed out over Lands End before turning south and making landfall in Brittany, France. These wintering locations are in line with what is expected for wild Curlew breeding in Norfolk.
At time of writing, 7 of the 12 birds tagged in 2022 are still transmitting. We know from metal ringing data from the BTO that only about half of young Curlews will survive their first year of life; so our tagged cohort is about average, and hopefully reflects the survival of the cohort which is only colour-marked. We know one GPS tagged bird was likely predated in late August on the shores of the Wash near King’s Lynn. The remaining 4 stopped transmitting over the winter when low sunlight conditions meant the tag batteries briefly stopped charging, and these never came back online again, so the fate of these birds is uncertain.
Excitingly, the single GPS tagged bird still wearing her tag from 2021 (‘0E’) has recently transmitted from Breckland in Norfolk, an area with a known breeding population of approximately 150 Curlew pairs. While we think this bird has lost the aerial on her tag, meaning she only transmits locations occasionally, it’s thrilling to see that she has joined a nearby breeding population after spending most of her first 18 months of life on the south shore of the Wash.
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