Abstract
I studied the ecology of the goshawk-grouse relationship in
Oulu, northern Finland, during and outside the breeding season,
by radio-telemetry. This included museum samples of goshawk to obtain a
better ecological as well as a better evolutionary understanding
of it.
The proportion of grouse in the diet of goshawks has decreased
since the 1960's, in accordance with the decline of grouse
populations. The main prey groups replacing the lacking grouse were corvids,
squirrels and hares. The proportion of grouse was highest in spring
and it decreased towards the end of the nestling phase. The most
preferred grouse species were hazel grouse Bonasa bonasia and
willow grouse Lagopus lagopus. Preferences
for different prey types are not explained by active choices of
goshawk, but by changes in the vulnerability of the prey species.
The nestling phase, when food demand is highest, is not adjusted
to when prey supply is highest, but before it.
The size and shape of the goshawks has changed from the 1960's.
Adult males became smaller but females larger. Both became relatively
longer winged and tailed. Decrease of male's size may be
a response to the change in the food supply. Prey types replacing
grouse are generally smaller, which may cause the change in the
male's morphology. Females being less active during the
breeding season may not be affected. For the female to be larger
is advantageous in winter when they kill 'over large' prey
like mountain hares Lepus timidus and capercaillie
cocks Tetrao urogallus.
Wintering goshawks were mainly females in adult plumage that
tended to stay in the study area. However, only one third bred locally.
More than one quarter of all hawks died during the study. Although
known to be inhabitants of old forests, which this study supports,
goshawks are fairly well adapted to mosaic landscape resulting from
modern forestry, providing that suitable sized prey is available.
Females have less problems, probably because hares, the main winter
prey for females, are not affected negatively by forestry, like
grouse and squirrels are, the main prey for males.
Goshawks have a remarkable impact on grouse populations, especially
when non-territorial hawks, 'floaters' are also
included. About one half of the total mortality rate of grouse may
be due to goshawk predation. Goshawk predation accords to predictions
of general predation theory and may be a noticeable factor contributing
to cyclicity in grouse.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-5637-9 |
| Date | 03 May 2000 |
| Creators | Tornberg, R. (Risto) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2000 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3191, info:eu-repo/semantics/altIdentifier/eissn/1796-220X |
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