Individual study: Provision of artificial cover for common guillemots Uria aalge nesting on a cliff top at Tatoosh Island, Washington, USA
Parrish J.K. & Paine R.T. (1996) Ecological interactions and habitat modification in nesting common murres, Uria aalge. Bird Conservation International, 6, 261-269
The common guillemot (or murre) Uria aalge is a highly gregarious seabird. In Washington State, USA, the species breeds annually at only one location, Tatoosh Island (48º24' N, 124º44' W), where it nests in two types of habitat: natural crevices in vertical cliff walls; and open space on the cliff top, adjacent to and underneath 1-2 m high stands of salmonberry Rubus spectabilis. Old photographs indicate that, although the crevice subcolonies were already completely colonised, the main cliff-top subcolony on Tatoosh did not exist prior to 1980, when the salmonberry cover extended right up to the cliff edge. However, since the colonisation of the cliff top by guillemots in the 1980s, the area of bare ground has expanded at a rate of around 26 mÂ² per year, with the salmonberry cover dying back as a consequence of guano over-fertilisation. By the mid-1990s, around 75% of the population nested on the cliff top, but the increasingly exposed nature of the habitat led to a dramatic reduction in reproductive success as a consequence of egg predation by glaucous-winged gulls Larus glaucescens and northwestern crows Corvus caurinus, facilitated by disturbance from overflying bald eagles Haliaeetus leucocephalus. This study documents an attempt to increase the amount of cover for nesting guillemots, and hence improve their breeding success, by providing artificial â€œsilk treesâ€ within the cliff-top subcolony.
In April 1994, before the start of the breeding season, two grids of artificial silk-enhanced â€œtreesâ€ were installed in an area of the cliff-top subcolony immediately adjacent to the live salmonberry, which had been used by nesting guillemots in previous years. The grids (3.25 m² in size) were positioned 2.6 m apart, and were made up of artificial trees placed at 40 cm intervals. Each tree was constructed from a 90 cm stake, driven into the ground to a final height of 75 cm, with a set of artificial branches affixed to the top with tie-wraps. Artificial silk leaves extended 5 cm above the stake in a 25 cm radius, providing almost total cover (similar to that provided by live salmonberry).
During 17-26 June and 8-11 July 1994, reproductive output, guillemot presence and the response of adults to overflying bald eagles were monitored from a promontory c.150 m away from the subcolony. The number of guillemot eggs was counted during c.50 hours of observation as well as three visits to the subcolony during its temporary abandonment by birds. Data on egg production (measured as the number of eggs per m²) were collected for two areas: i) the modified area, 11.2 m² (24% of the total nesting area) comprising the two artificial silk forest grids and the area in-between them; and ii) a control area of 18.8 m² (40% of the total nesting area) of bare ground immediately adjacent to the modified area. Eggs laid in the area under the live salmonberry (c.17 m²) were not sufficiently visible to count accurately.
Guillemots colonised all available space in the subcolony, including the modified area, with the density of nesting birds apparently equal and maximal (i.e. shoulder-to-shoulder) throughout. During the 14 days of observation, egg production in the modified area (124 eggs; 11.1 eggs/m²) was almost twice that in the control area (108 eggs; 5.8 eggs/m²). In addition to higher productivity, guillemots nesting in the modified area spent more time at the colony than those nesting in the control area. Following overflights by bald eagles, about 85% of guillemots flushed on average from the control area, compared with around 5-30% in the modified area. Nevertheless, the cliff-top subcolony failed to produce chicks, even in the modified area, during the 1994 season, possibly because the silk forest covered only a minority of the total nesting area.
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