2.1.3 Acoustic surveys

Acoustic surveys often follow up on historical records, or from current reports of seabirds. These could be observations of birds during the day or, in the case of many Procellarids, birds flying over and calling at night. In the simplest form it is a case of surveyors heading out at night, going to locations where birds have been reported and listening for their calls. Human hearing can be very acute, allowing a listener to get an idea of the direction and distance from where calls are coming from. Or, if the birds are flying over, observers may ascertain the direction in which they are travelling. These observations can lead to further investigation using acoustic sensors (see next section).

Nocturnal Surveys

Many Procellarid species are only active at their colonies during the night, spending the days far out at sea hunting their marine prey. This presents particular challenges for surveying, as well as safety considerations. However, undertaking nocturnal surveys is an important facet of studying these species, as these surveys help to locate breeding colonies and active burrows as well as focus management actions to core activity areas. Nocturnal surveys often include both an auditory component (such as tallies of calls) and a visual component (actual sightings of birds, using specialised equipment like night vision and thermal binoculars/scopes).

It is a good idea to split surveys into discrete sessions (30-minute sessions are a good starting point) and include time within each session to collect weather data. Inclement weather conditions such as wind and rain can impact on the observer’s ability to hear distant or faint calls and in some cases may mean there is no point in surveying. Keeping detailed data on the weather conditions throughout your surveys allows you to gauge the reliability of your data later and compatibility with surveys at other locations. For the auditory component, identifying what you are considering as a single data point is also key – classify beforehand what a ‘call’ is (i.e., a single unbroken note or series of notes). Throughout your surveys, include data on whether your data points relate to auditory or visual records. For each record, collect as much data as possible, including time of observation, species, direction from observer, distance from observer, and for visual records the behaviour of bird (with particular attention paid to activity indicative of breeding, such as circling behaviour and ground-calling). Recording data collection events where no seabirds were detected is also useful, as knowing when birds are not around is equally important.

At the end of each survey, it is also useful to draw polygons of seabird activity on a map (print out detailed topographical maps of your survey site beforehand and bring it with you). Categorise your polygons into levels of intensity (e.g., high, medium and low), draw arrows indicating transit routes, mark on the map where ground-calling was recorded etc. The more detailed it is, the more useful your map will be for guiding burrow searches or management actions.

Some important considerations

• To carry out nocturnal surveys, it is important to familiarise yourself with your survey area during the day – look at the terrain around your survey location and assess where key features are (ridges, clumps of trees, drainages) relative to your position and how far away they are. This will allow you to better assess where birds are calling from or circling once it gets dark, as well as allowing you to make a safety assessment of the area for when you are moving around after nightfall. Make sure you are in position before it gets dark. Some species begin coming into colonies before dark, so again an understanding of the behaviour of your study species is key.
• Understanding when your species are active in colonies is critical – there is no point conducting surveys during a time of year or a time of night when the birds are not at the colony! If you do not know this, then carry out surveys at multiple points during the night (particularly the first few hours after dark and the first few hours before dawn, which are often periods of high activity within colonies for many Procellarid species). Literature searches could also provide you with useful information on directing your surveys, particularly in terms of breeding phenology data. Colonies are often particularly busy when birds first return to the colony and when sub-adults and prospectors are in attendance. Activity can often drop off as birds start incubating, and vocalisations can decrease when adults are feeding young. For many species there is a marked increase in non-breeder activity around hatching dates as the birds evaluate whether the colony is successful and therefore worth joining.
• Storm-petrels calls are often relatively quiet, and some species may only call frequently when they are on the ground. Conversely, some species can be selected for only aerial calling, for example, New Caledonian Gould’s Petrels (Pterodroma leucoptera caledonica). This could be a result of the impact of cat predation which has largely killed off the population, so there are fewer birds on the ground vocalising.32
• Many Procellarids respond to moon phase, with birds becoming quiet during the week before full moon (presumably a predator avoidance tactic). Use a lunar calendar as part of your survey planning process.
• The number of birds in a colony can also impact data collection. For a rare species, where there are only a small number of calls or sightings to record during your survey, you need to collect as much data as possible. However, in large colonies, it may not be practical to collect detailed data on every observations – large petrel and shearwater colonies can be overwhelming in the level of noise and activity. In these scenarios, you can reduce the amount of data collected for each sighting (focus on the most important bits of data you need), utilise multiples (i.e., sightings of 10, 20, 30 birds etc), collect data using a counter, etc. If you are using night vision or thermal binoculars, consider breaking your survey sessions down into portions where only auditory contacts are collected and those where you use your night vision. For example, in a 30 min session, you could collect weather data for 5 minutes (which also gives you a chance to pause in frantic seabird data collection), 10 minutes for night vision, and 15 minutes collecting auditory data only.