Wednesday 25 January 2017

Hen harrier tagging project roundup 2016

Hen harrier productivity and nest success

A total of 39 sites were monitored across all regions as part of the satellite tagging projects with the highest fledging success rates in the Ballyhouras and the lowest in the Slieve Blooms (Table 1). A high failure rate in the Blooms, with three pairs failing early in the season and predation proven or suspected in another four cases, contrasted with good nest success rates estimated for the Ballyhouras and Duhallow regions. 

Sites checked
Sites occupied
Successful pairs
Mean brood size
Fledged juveniles
Mean fledged/ nesting pair
Mean fledged/ successful pair

Satellite Tagging in 2016

Ten harrier chicks were tagged at nests spread across the Slieve Blooms (2), Knockmealdowns (1), Ballyhouras (4), Boggeraghs (1) and Mullaghareirks (2). These included 5 males and 5 females from broods of 2-4 chicks. The tagged chicks were estimated to range from 24-35 days old. Chicks were sexed based on tarsus width, weight and/or iris colour.

Post-fledging dispersal

Six of the 10 satellite tagged juveniles dispersed away from the natal site (Fig. 1). Fledged satellite tagged juveniles began to explore beyond the local nest site (>1km) from 18-35 days after tagging and dispersed away from their natal areas 21-40 days after tagging.  Most juveniles (i.e. four of the six) dispersed, at least initially, in a northerly direction while two dispersed is a southerly or easterly direction.

The Boggeraghs juvenile (160250 female, green track) was somewhat exceptional in leaving her natal site in mid-July, 18-21 days after tagging. This equates to an approximate age of 51-54 days old. This was at least partly explained by the fact that this was the most advanced nest, over three weeks earlier than the last tagged bird (160251). In comparison, other juveniles dispersed away from natal sites in early to mid-August: Slieve Blooms (159138 female, 52-59 days, red track; 159142 male, 62-65 old, blue), Knockmealdowns (159143 male, 62-66 days, yellow), Ballyhouras (160250 female, 57-59 days, purple) and Mullaghareirks (160251 female, 58-63 days, black). 

Summary of post-fledging survivorship

Of the 10 tagged harriers four apparently died within two weeks of fledging. Two were recovered 200-250m from nests having apparently died 6-14 days after fledging. Another (160248) died some 19 days post-fledging although this bird had dispersed early away from its natal site. Another four birds survived 54-72 days post-fledging but died within a narrow window (18-22 September) in different locations. Of these four, 160251 may have died due to a wind-turbine strike. However, it should be noted that this bird had at least background, and possibly sub-lethal, levels of brodifacoum, a 2nd generation rodenticide. The other three harriers died over a 1-2 day period. Met Eireann notes that the September rainfall values were above the long-term average with exception of a few isolated stations away from the locations of the three harriers. Thus, these birds may have succumbed to a period of prolonged rainfall in combination with poor body condition as a result of difficulties in finding enough food.  The final bird (159142) was recovered dead in late October, but the post-mortem was inconclusive.

What we have learned so far

- Almost 500 tracking days from 10 harriers have been collected resulting in the acquisition of over 2,500 fixes providing information on the activity of the subject birds;

- The majority of these fixes (almost 1,500) are of a high enough quality that can be used to help identify dispersal patterns as well as important post-breeding roosting areas; 

- Empirical evidence of pressures acting on this species in Ireland was gathered including possible secondary poisoning and wind farm collision incidents; and

- High rates of post-fledging mortality have been estimated reinforcing earlier studies but sample sizes remain relatively small.

While the loss of all juveniles before their first winter reiterates the high mortality rates of young hen harriers, such mortality if extrapolated to the wider population would be clearly unsustainable. However, although the sample size of tagged birds remains small, and the high mortality experienced here may have been in part due to hatching order effects (lower survivorship of last hatched chicks), young harriers clearly face many threats and pressures. We hope to be able to be able to understand these better through satellite tagging.

Thursday 22 September 2016

Ríaghán (male, tag no. 159142)

Ríaghán was born in a nest surrounded by tall heather in the Slieve Bloom Mountains Nature Reserve. Nearly all the harrier nests in the Slieve Blooms are in heather moorland and blanket bog. The female’s speckled brown plumage blends seamlessly into the heather when she is on the nest. This camouflage is essential as the nest is extremely vulnerable to predation and disturbance and some nests unfortunately fail for these and other reasons.

Ríaghán in the nest – note the antennae extended from the tag

A nest camera was installed under NPWS license to monitor the progress of Ríaghán and his sister, and to capture any possible nest predation. Frequent food deliveries by the male and later by the female ensured that this brood grew up healthy and strong and both successfully fledged the nest. Prey items were mainly small passerines (mostly meadow pipit) and small mammals. The female was also captured on camera feeding a frog and a young rabbit to the chicks.

Dinner time!  - adult female feeding a frog to Ríaghán’s siblings

The nest was near an area of felled invasive conifers on the blanket bog (part of the NPWS conservation management of the site) and the remaining tree stumps were frequently used as perches by the females and, later, the juveniles. Bad weather in late June meant that the female had to spend long periods brooding the chicks. Luckily, they had most of their plumage at this stage to survive as she intermittently left the nest to go hunting for food. Many nests in the Slieve Blooms with younger chicks did not survive this year.

Ríaghán and his sister enduring the wet weather. The adult female returned to nest with food and brooded them moments later

Ríaghán was fitted with a satellite tag on the 5th of July 2016. He was named Ríaghán which means ‘little king ‘ in Irish as it suited the regal demeanour of this bird who almost always seemed to be surveying his surroundings when viewed through a telescope. Ríaghán also denotes the ancient territory of Ui Ríaghán which encompassed the part of the Slieve Blooms where he was born and also the medieval clan of the same name who lived here. It is hoped that he will return again to his kingdom someday but future posts will follow his journey away from his natal site.