3.11 Case study - The effectiveness of predator control

Assessing the effectiveness of predator control in endangered seabird management sites on Kaua’i using acoustic sensors

Acoustic sensors have been deployed in seven management sites on Kaua’i for two endangered seabirds – the ‘ua’u (Hawaiian Petrel) and ‘a’o (Newell’s Shearwater) – for over a decade to assess the efficacy of colony management actions (mainly the control of introduced  predators and invasive plants). Each year, acoustic sensors (Wildlife Acoustic SM4 units) are deployed at 10-14 static survey locations within each site. These are spaced 250 meters apart to prevent double counting of birds (the distance is based on how far the species calls typically travel and are deployed in such a way as to provide auditory coverage over the majority of each of the sites). This long-term monitoring approach allows for a colony-wide assessment of inter-annual vocal activity within each colony.

To ensure that units capture the peak vocal period of both species, units are deployed for a five-month period between May and September, capturing critical periods of the species breeding phenology: courtship, incubation and early chick-rearing periods. As the vocal activity patterns of the two species are markedly different, units are set to record every night at the following rate: 1 minute in every five minutes for 5 hours after sunset and 1 minute in every ten minutes for 5 hours before sunrise. Call rates are calculated for ‘ua’u based on data collected in the 5 hours after sunset (this species is not vocal after the first few hours after dark) and for ‘a’o based on data collected in the 5 hours before sunrise (this species is vocal in both periods, but more vocal in the hours before sunrise).

Figure 126. ‘Ua‘u and ‘a‘o call rates recorded by acoustic sensors at two different management site on Kaua’i. Supplied by André Raine.

As call rates for both species have previously been found to be directly related to the number of burrows present around each acoustic sensor (i.e. higher call rate = more active burrows), call rates are used as a proxy for assessing colony size. Management (predominantly predator control of introduced cats, rats and pigs) at each site began at the time monitoring commenced, was refined over the first few years of monitoring and has remained at the same intensive level since. Results from acoustic sensors have clearly demonstrated the positive impact of these management actions by documenting a steady near annual increase in call rates (as shown in Figure 126). Acoustic monitoring on an annual basis at these sites has provided a standardised long term and cost-effective method that allows land managers and decision makers to easily assess the effectiveness of their actions.

This mirrors results from other monitoring work undertaken at each colony. In tandem with acoustic sensors, all known burrows (over 1,400 burrows across all sites) are visited eight times a year to assess reproductive success rates, and at each site 30 monitoring cameras are deployed on a sub-set of burrows to assess predator visitation rates and predations. These additional methods are in line with the results found from the acoustic sensors, with increased reproductive success coming in tandem with decreased predation rates and predator visitation over the same time period. As these results show, acoustic monitoring is a cost effective and low intervention method of monitoring the success of management interventions such as predator control.