| APPENDIX 2. |
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| Foraging
ecology of sawbill ducklings in North East Scotland. |
| |
| As part of
the current investigation, a stand-alone study of the
diet and foraging behaviour of sawbill ducklings was
carried out (1992-94) as a PhD study (Alexander 1995).
This work, jointly supervised by the Institute of
Terrestrial Ecology and the University of Aberdeen,
considered populations of ducklings and fish communities
on the Aberdeenshire Dee (goosanders) and North Esk
(mergansers). Full details are given in Alexander (1995),
a summary of the main findings is given here. |
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| Fish
population sampling |
| |
| Fish
populations were assessed by a single electrofishing
session within a 15 x 3 m quadrat located along the
river's edge. Electrofishing catches (n = 60) from the
North Esk comprised, in order of numerical abundance,
salmon, stoneloach, minnow, three-spined stickleback,
brook lamprey, eel, trout and a single flounder, those
from the Dee (n = 43) comprised minnow, salmon, eel,
brook lamprey, trout and stickleback. The spatial
distributions of these fishes (upper, middle and lower
mainstem) were similar on the two rivers, the main
exception being the absence of stoneloach on the Dee. In
terms of total biomass, salmon was the dominant species
in samples from both rivers, with the exception of eel in
the lower stretches. The proportion of salmon in samples
increased with distance upstream in both rivers, although
the total number and size of fish decreased. |
| |
| Diet of
sawbill ducklings |
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| The stomach
contents of sawbill ducklings were analysed in a similar
manner to those of adult birds detailed earlier (Chapter
3). Stomach contents were available for 34 goosander
ducklings, representing 23 broods from the Dee. Twenty
five individuals, representing 17 broods, contained fish
remains, the rest contained either invertebrate remains
or were empty. A total of 154 fish (estimated mass =
650g) were recovered from stomachs, most (86 % by number,
84 % by mass) were salmon, the remainder trout, minnow
and brook lamprey. A few of the salmon (14%) were
young-of -the-year but most were older. Comparison with
electrofishing catches suggested that goosander ducklings
on the Dee selected against minnow and brook lamprey but
preyed unselectively on salmon and trout. |
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| Stomach
contents were available for 18 merganser ducklings,
representing 9 broods from the North Esk. Sixteen
individuals, representing 9 broods, contained fish
remains, the rest contained either invertebrate remains
or were empty. A total of 250 fish (estimated mass =
317g) were recovered from stomachs. Three-spined
sticklebacks were most numerous (57% by number, 11% by
mass) although salmon were greater in terms of mass (57%
by mass, 32% by number). Trout and minnow were also
recorded but in smaller quantities. In contrast to
goosander ducklings on the Dee, most of the salmon (97%)
taken by merganser ducklings on the North Esk were
young-of -the-year. Comparison with electrofishing
catches suggested that merganser ducklings on the North
Esk selected against minnows but preyed unselectively on
salmon, trout and three-spined stickleback. |
| |
| Foraging
habitats |
| |
| Time
activity budgets were constructed from visual tracking
sessions of duckling broods which, wherever possible,
lasted 24 hours and included all daylight activity and
the overnight roost period. These showed that both
goosander and merganser ducklings spent most of their
time foraging and loafing with most foraging occurring in
the mornings and evenings. Both species spent most of
their foraging time (about 65-70%) feeding along the
shallow edges of the river although goosander ducklings
spent increasing proportions of their time foraging in
deeper, faster-flowing areas of water as they became
larger. Neither species showed any preference for riffle
or pool habitat. |
