Raymond Klaassen is one of the lead researchers at the Dutch Montagu's Harrier Foundation. Here he tells us about his work using satellite tracking to study the migration and mortality of Montagu's harriers on the continent. 

This nomadic species is a close relative of the hen harrier and so similar in appearance to the untrained eye, it can be difficult to tell them apart. Montagu's harriers currently breed on agricultural land in just three locations in the UK, and widely across Europe, from Spain to Belarus. The satellite tags used by the Dutch Montagu's Harrier Foundation are of the same make and model as those currently being used to track hen harriers in the UK. The tagging process is also subject to the same stringent licensing procedures to ensure the welfare of the birds always comes first. 

The Montagu’s harrier is a rare breeding bird in the Netherlands with a breeding population of about 30-60 pairs. Conservation includes fencing all nests in agricultural fields in order to protect the young during harvest. In addition, efforts are made to improve the harriers’ foraging conditions via Agri-Environment Schemes. However, as Montagu’s harriers are long-distance migrants wintering in sub-Saharan Africa it is equally important to also conserve this elegant species during the non-breeding season. If for example a disaster would occur along the migration route or in the wintering area, all conservation efforts on the breeding grounds would be in vain.

A basic but essential step towards a year-round conservation is to determine migration routes and wintering areas and satellite tags are the perfect tool with which to do this. In 2006 we tagged the first two Dutch Montagu’s harriers using satellite transmitters, and it was thrilling to be able to follow the journeys of the birds via the daily updates. Since then, we have tracked more than 67 adult Montagu’s harriers from six different countries in Europe, in which the UK has been the latest addition.

Satellite transmitters are small technological wonders that allow tracking individual birds around the globe in almost real-time. This is vital, as it actually allows studying when and where the birds die. It is always sad to lose a bird that you have come to know quite well, but information about mortality is of course extremely important for conservation. After having accumulated seven years of tracking data, we decided to review the causes of death of all of our tracked birds to date. In order to boost the dataset we also included data from Swedish Marsh Harriers and Ospreys which had been satellite tracked by colleagues from Lund University in Sweden using the same technology.

One important question we had was whether migration is a dangerous behaviour in comparison to breeding and wintering. Indeed, we lost relatively many birds during time they were travelling, and thus the daily mortality rate was clearly raised for migration periods, especially for spring migration. The safest time of the year turned out to be the winter in Africa.

When a bird dies, the transmitter is designed to keep sending positions, providing a large number of data points from the final location. The satellite transmitter also has an activity sensor which indicates whether the bird is moving, and this sensor data can be used to confirm the death of the bird. Mortality is more difficult to prove when contact with the transmitter ceases abruptly (observed in 14% of all cases). Was it the bird that died or has there been a technical failure of the transmitter? Technical failures generally are rare. We have recorded a few throughout the years (6% of all cases), however failures have always been preceded by irregular transmission periods and, most importantly, a drop in battery voltage (another parameter monitored by the transmitter). This makes it relatively straightforward to distinguish between a likely mortality event and a likely transmitter failure. Indeed, we never saw a bird returning to the breeding area that we had deemed to have died based on the different sources of satellite telemetry data, but we have seen birds returning with non-functioning transmitters in cases where we had deemed technical failures.

A sad but instrumental example of how satellite telemetry could help to evaluate individual cases of mortality is the disappearance of Montagu’s harrier female “Mo” in East Anglia in 2014. This breeding bird was tracked successfully for several weeks after tagging, until suddenly no new locations were received after the 8th of August. Technical failure could readily be ruled out in this case as the transmitter had been working perfectly well up to the point contact ceased (and Mo was not observed in the field anymore despite extensive searches). Most likely the bird died but it is unlikely that a natural predator was involved given the fact that the signal stopped so abruptly. In the event of a natural death, we would expect the tag to continue transmitting and send out a new signal to indicate the bird had died. In fact, this information combined by the fact that the last positions were received from a hunting estate points towards illegal persecution.

In summary, satellite telemetry actually is a powerful tool to prove illegal persecution. For example, the repeated disappearance of tagged Hen Harriers and Golden Eagles in certain areas in the UK can only be explained by high levels of illegal persecution. The use of this technology opens exciting opportunities to not only study natural causes of mortality of raptors in the field in more detail but also to fight illegal persecution in a better way.

Suggested reading

Klaassen, R. H., Hake, M., Strandberg, R., Koks, B. J., Trierweiler, C., Exo, K. M., ... & Alerstam, T. (2014). When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors.Journal of Animal Ecology83(1), 176-184.

Koks, B. J., Trierweiler, C., Visser, E. G., Dijkstra, C., & Komdeur, J. (2007). Do voles make agricultural habitat attractive to Montagu's Harrier Circus pygargus?. Ibis149(3), 575-586.

Trierweiler, C., Koks, B. J., Drent, R. H., Exo, K. M., Komdeur, J., Dijkstra, C., & Bairlein, F. (2007). Satellite tracking of two Montagu’s Harriers (Circus pygargus): dual pathways during autumn migration. Journal of Ornithology,148(4), 513-516.

Trierweiler, C., & Koks, B. J. (2009). Montagu’s harrier Circus pygargus.Living on the edge: Wetlands and birds in a changing Sahel, 312-327.

Trierweiler, C., Mullie, W. C., Drent, R. H., Exo, K. M., Komdeur, J., Bairlein, F., ... & Koks, B. J. (2013). A Palaearctic migratory raptor species tracks shifting prey availability within its wintering range in the Sahel. Journal of animal ecology82(1), 107-120.

Trierweiler, C., Klaassen, R. H., Drent, R. H., Exo, K. M., Komdeur, J., Bairlein, F., & Koks, B. J. (2014). Migratory connectivity and population-specific migration routes in a long-distance migratory bird. Proceedings of the Royal Society of London B: Biological Sciences281(1778), 20132897.

Schlaich, A. E., Klaassen, R. H., Bouten, W., Both, C., & Koks, B. J. (2015). Testing a novel agri‐environment scheme based on the ecology of the target species, Montagu's Harrier Circus pygargus. Ibis157(4), 713-721.

Vansteelant, W. M. G., Bouten, W., Klaassen, R. H. G., Koks, B. J., Schlaich, A. E., van Diermen, J., ... & Shamoun‐Baranes, J. (2015). Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales. Journal of Avian Biology,46(1), 25-39.

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