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.
Region
|
Sites
checked
|
Sites
occupied
|
Successful
pairs
|
Mean
brood size
|
Fledged
juveniles
|
Mean
fledged/ nesting pair
|
Mean
fledged/ successful pair
|
Blooms
|
10
|
10
|
3
|
3.2
|
6
|
0.9
|
2.0
|
Ballyhouras
|
11
|
8
|
6
|
3.0
|
15+
|
2.1
|
3.0
|
Duhallow
|
14
|
12
|
8
|
3.0
|
17+
|
1.5
|
2.1
|
Knockmealdowns
|
4
|
3
|
1*
|
2.5
|
4
|
1.3
|
-
|
All
|
39
|
33
|
18
|
2.9
|
42+
|
1.4
|
2.5
|
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;
- 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.