"Finland has a beautiful resurrection story about white-tailed eagles," says Markus Varesvuo. "They were almost extinct here in the 1970s because of toxic pollutants. Then a group of people started an ad-hoc feeding programme and soon the World Wildlife Fund in Finland joined them. Over 40 years later the bird is doing very well. What used to be a rare sighting is now what it should be: a part of the coastal landscape."

The white-tailed eagle is one of the largest birds of prey, with a wingspan of up to 2.45 metres (8ft). It usually lives near open bodies of water and feeds mainly on fish. To catch them, it swoops down, snatching them from the water surface with its talons.

What particularly fascinates you about birds?

"It's not any one particular thing, more like a life-long love affair, and it's showing no signs of waning. Flight is a big attraction, of course – take a hawk for example, flying in the middle of the forest after prey. The way it manoeuvres through the trees and branches is amazing. How a gannet falls from the sky and pierces the water, how a gull mobs an eagle, how a waxwing works a rowan tree clean of berries. The challenge of capturing these things with a camera is a thrill that never ceases to fascinate me."

What are the main challenges of photographing birds?

"Well, they tend to fly away. Or high. They are skittish. It's a game of hide and wait, requiring tons of patience, yet action is just around the corner and needs fast reflexes. Birds are outdoors, so it's a constant negotiation with the elements – rain, fog, cold, heat, bright sunlight and darkness. One can't be lugging all the equipment around, so it's always a compromise, [and] you work with what you have. So many choices are made by forces outside your control. The equipment today is so good that gear is pretty low on the list of challenges."

Why do you use the EOS-1D X Mark II for bird photography? And what lenses do you typically use?

What, for you, are the greatest improvements in the EF 600mm f/4L IS III USM (over the Mark II)?

"The loss of almost a kilo, together with an improved weight distribution, makes it a fantastic lens to work with in handheld photography, which I do a lot. And this has been accomplished with no loss of quality. Impressive!

"It performed perfectly for photographing the white-tailed eagle. The EF 600mm f/4L IS II USM is an excellent lens, and with the Mark III I get all its features plus improved manoeuvrability, which spares my back and shoulders some considerable strain. Working all day with white-tailed eagles in handheld shooting from a boat, the light weight is a delight."

Where did you photograph the white-tailed eagle?

"I've been photographing white-tailed eagles with Ole Martin Dahle, the 'Eagle Man' in Flatanger, Norway, for over 15 years. Every trip produces new, exciting material. We go out in his boat and work with the animals, the elements and the landscapes."

Are these birds difficult to find and photograph?

"They aren't easy to photograph as they fly high and far away. The best way is to photograph from a hide positioned near a feeding place, or from a boat at sea as they descend from height to get some fish tossed from the boat. Like gulls, white-tailed eagles learned a long time ago to associate fishing boats with food."

How did you get such great action photos of eagles snatching fish?

"We were out on four days, from morning light until the end of the day, taking a break at midday. These kinds of shots are made in cooperation with a knowledgeable guide and confident birds that have learned to make good use of a reliable source of food. This requires patient, systematic work from the guide – it's not something that can be done only on the days when there's a photographer. Ole Martin has worked with the eagles in his region for 15 years; he knows them and they know him. It's done with the birds in mind first and foremost."

How did you get them in the right position to shoot?

"It's more about having the eye to see the potential in each situation and landscape, and a skipper who knows how to work their boat in the prevailing wind and weather and who also knows how each bird likes to have things. They are individuals with differing hunting styles and strategies."

You shot in a huge variety of light conditions. How did your kit cope with that?

"I used evaluative metering and where I saw that the light was especially difficult, I compensated exposure. I've been photographing birds, with Canon gear, for so long that how I compensate comes to me automatically. It's situational, instinctive, sort of in my DNA. Whatever metering option you use, you really need to know how it works."

How did you capture that spectacular lead image, where the low golden light on the water droplets makes them look like glitter?

"The location is the key here. It's late autumn light, coming at a low angle over a hillside onto the fjord where we were in a boat. The background behind the eagle is the water surface and a dark hillside, darkened by being in the shadow. So the converging elements – the low angle beam of sunlight and the rest of the world in darkness – create a strong backlit scene where the light hits the water droplets raised by the eagle as it grabs the fish in mid-flight and pushes back upwards into the air. There's added spray from a gull nearby that had its eye on the fish but took off as the eagle came in.

"It's a combination of light and the birds and the action – very fast situation, perfect light, optimal location, a touch of luck and the comfort and confidence that comes from knowing exactly how your gear functions."

In the sequence that image came from, how did you ensure the eagle was sharp in each shot, with a beautifully blurred background?

"Long lens, eagle full in frame. Dealing with very fast action means I need at least 1/3200 sec to freeze the action. With the aperture at f/4.5, the background is blurry, which is what I wanted.

"I used AF with 62 points. The eagle is so fast in its dive that it's practically impossible to keep just a few focusing points on the exact right spot. This series of shots, capturing the eagle descending and grabbing the fish, is just short of 30 frames. Maybe half of them are sharp, which is really rather good from the camera, and the photographer, in these circumstances. The unsharp ones are due to the focus hitting the nearest wing at times – but luckily in the best frames the focus is where it should be, which is the head."

Finally, what's the secret to getting great bird pictures?

"Knowledge of birds. It sounds simple but it takes time and effort. Information can be found in books, but knowledge comes out of time spent in the field, by observing and learning from the birds themselves, and also other bird people. It's a reciprocal relationship, though, giving and getting. The more time spent out in the field, the more chances present themselves."