Habitat Critical to SurvivalHabitat Critical to Survival

Habitat Critical to Survival

Christmas Island Frigatebirds can experience great difficulty in becoming airborne and cannot take off from perches less than 3m from the ground (Gibson-Hill 1947). During most of the year the prevailing wind on Christmas Island is the south-east trade winds, and nests are sited preferentially in the lee of the wind or protected (Stokes 1985). Frigatebirds nest under the top branches of trees and require a site that is calm enough to allow them to land safely. In 1994 a few appeared to nest in one or two trees high up the cliff above the road to Smith Point in Flying Fish Cove (A. Dunn unpubl. data). Nelson (1975) noted that they prefer to nest in Sea Almond Terminalia catappa. A survey of the golf course colony in 1983 found that the two species T. catappa and Celtis timorensis comprise 65.5% of all nest trees chosen (Stokes 1985). Ficus sp. trees are also often used for nesting (Marchant & Higgins 1990), but are rarely used in the golf course colony (Stokes 1985). These nest tree species occur right around the island and yet the species nests only in a small area of the island.

Data collected in the mid 1980s by T. Stokes may provide better characterisation of preferred nesting habitat. Analysis of this data is underway. Christmas Island Frigatebirds appear to be more restricted in their choice of nest sites, tending to nest well down on the shore terrace (Gibson-Hill 1947, Nelson 1972), whereas Greater Frigatebirds will also nest on the slope or lip of the inland cliff or higher up on the terraces (Gibson-Hill 1947).

Given the limited data on habitat requirements, habitat critical to survival of the Christmas Island Frigatebird is defined as all nesting areas. The approximate boundaries of the nesting colonies are given in Figure 1. Sympathetic management of areas adjoining nesting colonies may be very important. Abbott’s Booby nesting colonies within 300m downwind of cleared forest have been shown to have significantly lower breeding success than colonies upwind of the cleared areas (Reville et al. 1990), and it is highly likely a similar result would be found for Christmas Island Frigatebirds. Increased wind turbulence around Christmas Island Frigatebird nest sites is a cause for concern. Applying a precautionary approach given the available information, clearance of vegetation within 300m of nesting colonies should be considered undesirable. Actions proposed in this plan will help to better define habitat critical to survival of the Christmas Island Frigatebird, including whether the dryers colony is still viable nesting habitat.