Capercaillie Tetrao urogallus relocation after increased disturbance
along woodland tracks
Wild vertebrates usually avoid ground disturbed by humans but
consequences for their distribution and density are uncertain. The local
distribution of capercaillie shifted after an increase in disturbance
along woodland tracks adjacent to an expanding Scottish village. We
surveyed the birds’ droppings before and after the building of 30 new
houses, and model the probability of finding droppings (Pf) in relation
to period plus two disturbance gradients – distance to a much disturbed
‘entry zone’ by the village (dE) and ‘distance to nearest track’ (dT).
Estimates of Pf are benchmarked to average Pf (Pfav) – a notional
scenario in which the birds’ distribution is unaffected by tracks.
Change between periods occurred mainly on a strip of ground centred on
tracks and averaging 80 m wide, where Pf fell from about 0.5 Pfav before
the development to 0.2 Pfav after it. By contrast, Pf on ground 120–260
m from tracks, under a third of the 273 ha main study area, remained at
about 3 Pfav throughout the study – indicating a net influx of
capercaillie displaced from ground beside tracks in both periods. No
capercaillie droppings were found in the entry zone. Beyond this zone,
throughout the study, Pf increased as tracks sparsened until dE
approached 400 m – whereupon track density and Pf steadied together.
Beyond 400 m, Pf remained depressed on ground near tracks (dT ⪅ 100 m).
New desire paths after the development caused the proportion of ground
where dT < 100 m to increase slightly, from 56% to 60%.
Birds on roughly half of a 50 ha refuge should be undisturbed by direct
effects of track-based activities – but, if increases in density caused
by displaced birds are also deemed disturbance, a refuge would need to
be over 3 km2 to keep half of it undisturbed.