The Mystery of Migration

Peter Cairns/2020VISION

It is getting to that time in the season when the ospreys start to leave Rutland and potentially travel over 3000 miles to their wintering grounds. But what do we know about migration? Read on to find out more.

It is getting to that time in the season when our ospreys will start to leave Rutland and begin their migration south to their wintering grounds in countries such as Gambia and Senegal. These past couple of weeks we have already seen two of the juveniles from the Manton Bay nest leave – the two youngest 082 and 083. 082 was last seen on the nest and in Manton Bay on Monday 3rd August and 083 on Sunday 9th August.

On average, juvenile ospreys will leave their nesting site a month after fledging, when they are around 82 days old (~12 weeks). 082 and 083, were 85 days and 88 days respectively, when they began their migration. We are expecting 080 and 081 to leave any day now (they are 104 days and 102 days old) and 33(11) is continuing to frequently return to the nest with plenty of fish. Generally speaking, juvenile ospreys won’t fish for themselves until they leave for their migration, but will start to practice techniques and movements associated with fishing over the water and over land.

ospreys

Unlike some migratory species such as geese who migrate in family groups, individual ospreys will complete the journey on their own. This means, juvenile ospreys – who haven’t yet completed a migration – have to rely on their innate instincts to direct them southwards. It is yet unclear how juveniles, and adults alike, know when and where to go – it is the mystery of migration!

We know that birds migrate as a result of changes to their needs throughout the year, for example availability of essential resources for survival. Ospreys exclusively feed on fish and this plays a part in influencing their movements throughout the year. During the winter months the UK climate means there is a risk of water bodies will freeze over, potentially making it very tricky for them to fish. As well as this, during the UK’s winter, fish tend to remain close to the bottom, therefore are out of the ospreys’ reach.

But how do ospreys, and other birds alike, know when to go? For most birds, temperature and day length help determine when they should begin their migration - at the end of the summer the days begin to shorten and the temperature starts to drop.

In terms of the ospreys knowing how to get to their destination, there is still so much we don’t know or understand, but there are numerous theories. For example, we know many animals use their sense of smell to aid their navigation. But for those species, like the osprey, who travel much greater distances, it surely can’t be the only method they use. One theory is that there is a magnetite receptor on the beaks of some migratory birds, including ospreys, which is a mineral that helps them sense the Earth’s magnetic fields, in turn assisting them to navigate north or south. For those individuals who have already completed a few migrations, could they also be recognising geographical landmarks to help them get from their breeding grounds to their wintering grounds and vice versa? From looking at data received from our satellite tracked ospreys, we know they tend to visit the same stopover spots along the way and the routes they take each time they migrate mirror one another – they are crossing the English Channel and other geographical features in similar locations.