2.2.4 Colony surveying

Measuring population size and trends is fundamental to understanding the status and conservation needs of a species and designing effective management strategies. Population size is often measured first because many methods of measuring population trend require repeated measures of some aspect of population size. A complete census of all individuals in a population is very difficult to obtain, and in some cases only a portion of the population can be counted, such as breeders when they are in the colony. Furthermore, certain seabirds can be particularly tricky to obtain a population estimate, such as rare nocturnal burrow nesting Procellariforms.

Two case studies here illustrate surveys to establish population estimates and habitat use on a large scale for two species, in both cases whole islands, both in Aotearoa New Zealand.

The first, Te Hauturu-oToi/Little Barrier Island (hereafter Hauturu) is a 3000-hectare rugged island, whose highest point is 752m asl.

The second case study is for the Poor Knights Islands, where the two main islands were surveyed: Tawhiti Rahi (163 ha) and Aorangi (110 ha), with the highest point 216m asl. Both surveys’ estimates were based on habitat modelling and randomised quantitative sampling and represent repeatable assessments of the breeding population for both species. Major surveys like these also provide excellent opportunities to gather other data, provided that the expertise is available to conduct them (see some of the sections in Part Three).