Spotlight: Geolocators like “baby monitors” to study breeding behaviour

Scientists are resourceful creatures. Grant funding is tight, fieldwork is expensive, and it is often necessary to use field seasons and technologies for multiple purposes. This resourcefulness is nicely illustrated in a study¹ by Egor Loktionov, Pavel Tomkovich and Ronald R. Porter in this issue of Wader Study. The researchers used geolocators to investigate the breeding behaviour of Red Knots Caldris canutus rogersi in Chukotka, Russia. The geolocators were first employed to study migration pathways², but being resourceful, the researchers saw and seized an opportunity to use the technology to reveal important information on breeding behaviour. In essence, the researchers used geolocators like parents use baby monitors – to monitor breeding behavior without disturbing nesting shorebirds. But much like I watched my newborn sleep, while listening to the monitor to hear what rolling over sounded like, before the scientists could trust the technology, they needed to verify the signals it was sending.

What is a geolocator? Geolocators are light sensitive data loggers capable of storing light levels at regular intervals. Animal ecologists have generally used them to record the time of sunrise and sunset. Using the data stored on the loggers scientists can determine the location of animals wearing geolocators because day length varies with latitude while the time of solar noon varies with longitude (a handy trick that explorers have used to navigate for centuries).

Geolocators have been used to track the position of migrating birds for several years and scientists began using them on shorebirds in 2009. In shorebirds, geolocators are normally worn on the upper leg.

Serendipitously, when a bird is sitting on a nest and incubating eggs they shade the geolocator. Scientists can use this handy fact to gather data on incubation behaviour – and have already done so in the rufa subspecies of Red Knots³. Such data are especially useful in birds that breed in the Arctic and sub-Arctic where the nights are short (or non-existent) and, thus, a shaded geolocator is an especially informative tool for studying breeding behaviour.

Most Red Knots breed above the Arctic Circle in areas with continuous daylight in the summer. Thus for most breeding knots, the only time a geolocator would show darkness for a prolonged period is when a bird is sitting on a nest. However, Loktionov, Tomkovich and Porter studied the most southerly breeding of Red Knots, close to the Meinypilgyno Village in Chukotka. Meinypilgyno is below the Arctic Circle and thus there is civil twilight (when the sun is below the horizon, though there is enough light for objects to be distinguished under clear weather conditions) from approximately 10 pm to 2 am, even in high summer. This period of twilight at night contrasts with other studies that used geolocators to study breeding behavior in waders in the Arctic ^3,4 and the researchers called this four-hour period “nightshade”.

Because of the uncertainty introduced by nightshade, the researchers’ first goal was to confirm that the interpretation of light signals from the geolocators (incubation or off-duty) correctly represented actual behaviour. Although they were only able to observe two geolocator-wearing birds, their observations confirmed that they were correctly interpreting the light signals. Once in the breeding area, light levels fluctuated close to maximum most of the day. This changed about two weeks after arrival when the geolocators started to show prolonged periods of intermittent darkness and light, indicative of incubation and brooding. Such “ground truthing” was not possible in the earlier studies on waders.

Because the researchers were directly observing known males wearing geolocators, they were also able to define the incubation period in males at around 23 days and the period between when the last egg is laid and the chicks hatch at about 21 days. An earlier study on rufa knots of unknown sex³ showed incubation periods around 23 or 21 days. The ground truthing from this study suggests that birds with shorter incubation periods in the earlier study were likely to have been females (who incubate less at the start of incubation because they are still laying eggs). This difference in periods also correlates with the duration that birds stay on the breeding grounds – females leave soon after chicks hatch, while males stay to care for the chicks.

The authors also showed that that incubation bouts in knots were shorter in the first days of the incubation period and then increased in length. The reverse was true when knots were brooding chicks. Chicks seemed to need the most brooding when they were younger and when the weather was cooler (late June versus early July). Finally, this study showed that the low Arctic nesting Meinypilgyno males spend much longer on the breeding grounds after chick hatch than do their more northerly breeding counterparts.

The Red Knot is one of the most well-studied shorebird species on the planet. Yet information on its breeding biology remains sparse because incubating birds sit tight on their nests, making both birds and nests hard to find and because they nest in remote areas. In the six summers that researchers have been studying Meinypilgyno knots, only six nests have been were found (0-3 per season). Scientists who use the same technology as a means to multiple ends heighten the value of field seasons, and nests found during them. Resourcefulness and perseverance helped the authors of this study to verify geolocator inferred breeding behavior and then describe aspects of rogersi knot breeding biology. Thanks to studies like these, we have more confidence in geolocator data – just like I gained confidence in the baby monitor once I had seen for myself what the different sounds on the monitor meant.

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¹ Loktionov E.Y., Tomkovich P.S. & R.R. Porter. 2015. Study of incubation, chick rearing and breeding phenology of Red Knots Calidris canutus rogersi in sub-Arctic Far Eastern Russia aided by geolocators. Wader Study 122(2): 142-152.

² Tomkovich, P.S., Porter, R.R., Loktionov, E.Y. & L.J. Niles. 2013. Pathways, staging areas and incubation of Red Knots Calidris canutus rogersi breeding in southern Chukotka, Far Eastern Russia. Wader Study Group Bull. 120(3): 181–193.

³ Burger, J., Niles, L.J., Porter, R.R. & A.D. Dey. 2012. Using geolocator data to reveal incubation periods and breeding biology in Red Knots Calidris canutus rufa. Wader Study Group Bull. 119(1): 26–36.

⁴ Gosbell, K., Minton, C. & Fox, J. 2012. Geolocators reveal incubation and re-nesting characteristics of Ruddy Turnstones Arenaria interpres and Eastern Curlews Numenius madagascariensis. Wader Study Group Bull. 119(3): 160–171.