2.1.4 Acoustic sensors

Probably not relevant for Palau at this point.

Acoustic sensors (remote recorders) are very useful tools for seabird researchers who wish to locate or monitor colonies. They provide a way to record seabird calls and can be deployed for long periods of time, allowing studies to span critical periods of when target species’ breed, with data that are comparable year on year. They are particularly useful for nocturnal burrowing Procellarid species or large colonial ground nesting species such as terns.

Acoustic sensors can be utilised in four main ways in seabird studies:

Locating colonies of rare seabird species,
Comparing vocal activity between different colonies (as an inference of relative colony density, or times of year to assess when colonies are most active if breeding timing is unknown),
Used as a standardised measure of call rate change over time to assess management efficacy,
Assessing seabird collisions with powerlines and other similar infrastructure.

Method

Acoustic sensors should be deployed in areas where background noise (such as wind, waterfalls and rivers, roads etc) will least affect recording quality.

In colonies with little vegetation and limited rainfall, units can be set up on the ground and propped upright with rocks. If wind is an issue in this arrangement, a low rock wall can be built around the unit above the level of the microphone, keeping rocks clear of the microphones themselves, or you can install a foam windshield on the microphone itself. In areas with low ground cover but high rainfall, units can be mounted on poles driven into the ground. In these instances, units should be set at least a foot off the ground and attached securely to the poles using cable ties or bungee cords. If there is vegetation around the microphones, this needs to be removed to prevent noise from leaves or branches touching the microphones or banging against them in the wind. It is recommended that microphones are also covered by a rain guard (such as a length of guttering that shelters the microphones but does not interfere with their recording capabilities). Lastly, acoustic sensors can be mounted on trees, especially in areas with dense ground cover. Again in areas with heavy rainfall, a rain guard is recommended. Make sure prior to deployment that all necessary ports and vent covers are present, and that rubber lining is in good condition to discourage water intrusion.

Best practice is to deploy acoustic sensors in the same way across the entire study area, to ensure that acoustic sensor recordings are comparable between units. Other factors to consider for unit comparison is terrain and surrounding vegetation. For example, are all units positioned on ridges or slopes, are some units under tree canopy and others not, are some units in windier areas than others? All of these elements may affect the soundscape around a unit and thus comparisons between units. It is recommended that photographs are taken of all deployments and that standard measurements are taken of the deployment area including surrounding terrain features, vegetation (canopy, ground cover etc), distance to water sources such as rivers/waterfalls etc.). These kinds of measurements are vital for understanding results during the analysis stage.

Proximity of units to each other is an important consideration. An understanding of how far the calls of the target seabirds can travel (and be reliably recorded by the acoustic sensor) is critical for assessing how far apart units should be deployed and how many units are needed. To prevent double counting, the spacing of the acoustic units should be considered in the context of how far the species of interest calls travel in the environment (which can be identified during acoustic surveys as outlined in the previous section).

When to use

Understanding when units should turn on or off is an important aspect of acoustic sensor deployments, as there is a trade-off between collecting data and longevity of batteries. Having sensors turn on when birds are quiet and off when they are vocal would entirely defeat the objective. Understanding the vocal activity times of the study species is therefore very important. Likewise, the focus of the research plays an important role in deciding when to have acoustic sensors recording. If the focus is assessing the impact of management over multiple years in a busy seabird colony where there are lots of birds calling, then having the units record less data each night over multiple months allows for units to make the best use of battery life. In this scenario you may set the unit to record one minute in every ten minutes for three hours after sunset and three hours before dawn. On the other hand, if units are being deployed in areas to assess seabird composition or look for a rare species, then it would be better to record more data each night (to maximise the chance of recording a rare call) at a wider nocturnal spacing. In this scenario you may set the unit to record one minute in every five minutes from sunset to sunrise every other night, or even have it set on continuous recording throughout the night. Lastly, the location of study sites can affect what settings you may use. A site that is logistically easy to access and thus change batteries and SD cards regularly might be set to record more frequently than a remote and difficult to access site that can only be visited every few months. For the latter case, also consider using larger capacity SD cards so the units can store more data.

Figure 34. Examples of acoustic sensor deployments. Top left, on the ground on a dry poorly vegetated island. Top right, mounted on a tree in wet montane colony. Bottom left, on a tree in wet montane colony with dense understory. Bottom right, deployed from a helicopter in a specially designed deployment box. Photos: André Raine.

Analysis of data

Acoustic sensors can collect an enormous amount of data. For example, if one acoustic sensor is programmed to collect for one minute every ten minutes from dusk until dawn for three months, that equates to thousands of minutes of data. Then consider a project which deploys ten or twenty (or a hundred) units. Analysis of these data is therefore a critical factor when considering the use of acoustic sensors. There are several programmes that can be used to analyse acoustic data including Kaleidoscope by Wildlife Acoustics (https://www.wildlifeacoustics.com/products/kaleidoscope-pro) and the Cornell Lab’s Raven software (https://www.birds.cornell.edu/ccb/raven-pro/). There are also companies that specialize in the analysis of acoustic data (such as Conservation Metrics Inc https://conservationmetrics. com/), which may be a viable option depending on the amount of data to be processed and the amount of funding available. However, if you want to do a quick check of some of your recordings then a programme like Audacity is useful – https://www.audacityteam.org/.

Use of recordings

Recordings can be used for playback purposes including social attraction projects for locating burrows and for seabird restoration projects via playback (see Sections 2.1.5 and 4.4.2).

Additional information

Access to areas where the target species are breeding may not always be possible by foot – for example remote mountain tops, narrow dangerous ridges, etc. In these scenarios, if helicopters are available (both logistically and from a funding perspective), acoustic sensors can be deployed via grappling hook. Note that this is a very specialised deployment method that requires a highly capable pilot and should not be undertaken by those who do not have a significant amount of experience working in and around helicopters. Acoustic sensors can be mounted in specially designed boxes with handles (for the grappling hook) and stabilizers (to allow the unit to remain upright once deployed. Sensors are also recovered with the grappling hook.
For assessing collisions by seabirds with infrastructure, a clear understanding of how the sound of a collision is being created is necessary for the correct deployment of acoustic sensors, coupled with the surrounding soundscape. For example, assessing powerline collisions alongside roads may require mounting an acoustic sensor on the pole up into the wire array (and thus requiring the assistance of utility lines people), whereas assessing powerline collisions on spans crossing mountain valleys may involve mounting the unit on a pole at the base of the powerline pole itself. At a transmission tower with multiple guy lines, a single acoustic sensor at the base of the tower may be sufficient. However if there are a large number of guy lines that cover a large area, strategically placing multiple acoustic units may be necessary. Observing strike sounds in the field will help dictate acoustic sensor placement and is a critical component of any study using this technique to allow you to identify which species are colliding with the power lines. Also bear in mind that for units near roads, vehicle noise may be a confounding factor. Security of sensors placed in easy to access areas is also a consideration – to prevent theft or tampering with the units, consider making them harder to find (i.e., don’t leave them in an immediately obvious area) and consider the use of locks or theft proof cases.
There are several types of acoustic sensors available on the market, including Song Meters (http:// www.wildlifeacoustics.com) and Audio Moth (https://www.openacousticdevices.info/ audiomoth). It should also be noted that individual manufacturers also offer different options. For example, Wildlife Acoustics has SM4, SM Mini 2 AA and Li-ion, SM Micro and SM Micro 2. Not only do these units vary in terms of cost, capability, and battery life, but users should be aware that they may differ in their ability to record seabird calls. Therefore, projects should use only one type of unit across all colonies or years if comparisons are required; otherwise correction factors would need to be calculated.
New Zealand’s Department of Conservation produce lightweight acoustic sensors (Fig 35). These have a single microphone, record mono, and have two settings for high and low frequencies. High is generally used for bats. The low setting is very useful for seabirds and has the advantage of being a lot more economical on battery life. They have two time settings which is useful for nocturnal seabirds which call in the evening after sunset and in the hours before dawn. They use four AA batteries and recordings are stored in 15min sets on a single SD cards.
It is important to trial your recorders and get to know their capabilities, especially in terms of battery life, settings, and data storage, so you can service them regularly without missing calling periods.

Figure 35. One of the DOC acoustic sensors attached to a tree. The cage over the microphone protects it from being damaged (e.g. by parrots). Photo: Chris Gaskin