Literature review to assess bird species connectivity to Special ...

2. SPECIES ACCOUNTS
2.1 Red-throated diver
2.1.1 Foraging
Maximum foraging range
Red-throated divers usually fly to marine waters to feed. When breeding far inland,
they may occasionally feed on the nearest large body of fresh water (Cramp, 1977;
Bright et al., 2006).
A Scotland-wide study of breeding distribution in relation to the availability of fishing
waters found foraging trips to be limited to within 8km of the nest site, with the
exception of one site (Rannoch Moor) (Merrie, 1978).
A study on Lewis recorded outbound flights ranging up to 7.2km (mean: 3.7km; n=97)
from the breeding loch (Lewis Windfarm Environmental Statement, Appendix 12).
However, given the limitations in the survey method (fixed vantage points at breeding
lochs) this is likely to be an underestimate of the maximum range. Additional surveys
of pairs breeding on plateau moorland on Lewis recorded flight lengths of at least
11km, with maxima of up to 13.5km (Addendum to Lewis Windfarm Environmental
Statement, Appendix 12).
A study in Arctic Canada found that pairs nesting in excess of 9km from coastal
feeding areas were associated with reduced nesting success (Picman, 1993).
Non-breeding birds, and failed pairs, can congregate and show a large amount of
activity around lochans away from breeding areas. A study near Bettyhill found these
lochs may be 1km to 2km away from the Caithness and Sutherland SPA (Bettyhill
Environmental Statement).
Likely foraging destination
Feeding has rarely been recorded at Scottish breeding waters and occurs almost
exclusively at sea or in estuaries (Cramp 1977; Bright et al., 2006; Okill, 2007).
Likely route of foraging flights
In Shetland, many flights between the breeding loch and the sea are not direct, but
follow circuitous routes while over land. Jackson & Beasley (2006) considered that
such indirect routes were likely to be the most energetically economic for a number of
reasons:
they minimise the need for the birds to gain altitude to fly over high ground, by
following river valleys and passing through gaps between hills;
they avoid kleptoparasitism, i.e. stealing of resources such as food, nesting
material, etc, by skuas; and
wide, circling flights over the sea or adjacent land allow birds returning to
elevated nest sites to gain height gradually.
Divers’ characteristic long curving flights starting into the wind are a consequence of
the anatomical constraints of being heavy birds with very high wing loading and very
short tails (R Furness pers. comm. 2010).
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