On a boating holiday we had made the long climb though numerous locks from the Thames at Brentford and here we were on the Tring Summit, high in the Chiltern Hills.
Although it is just 30 miles north-west of London, the Grand Union canal at Tring can be a quiet and peaceful remote wildlife haven. Its days as the main commercial artery between the Midlands and the capital are long gone.
Now herons stand sentinel on the bank. A splash near the lock gate and a watery ripple indicated the water vole was about his business.
We had moored up on the far side of the canal against a high bank. Four of us had installed ourselves in the front cockpit of the narrow boat, and with drinks in hand were into the serious business of enjoying the warm and peaceful spring evening.
Suddenly an amazing electric spark, the brightest blue, missed my head by inches. What was this strange phenomenon? Then another spark, just as bright, just as blue, sent Ann scurrying for shelter in the cabin.
We were clearly under serious attack - but from what?
Then we realised. The blue flash was in fact Britain's most colourful, and in my opinion most beautiful bird.
It was a kingfisher (Alcedo atthis). But why this most uncharacteristic behaviour from what is normally a shy and secretive bird?
It was Sarah who spotted the tiny hole in the bank now effectively blocked by the bow of our boat. We had moored on the kingfisher family doorstep.
We manhandled the boat a few feet backward and sat quietly watching the tiny electric blue and orange bird arrive at the nesting burrow every few minutes with another wriggling silver fish in its long pointed beak.
Now we had got out of the way it wasn't disturbed or frightened by us in the slightest. It was clearly very busy feeding the young chicks in the nest.
As we watched we couldn't help but admire the courage and the protective instinct that had inspired this bird, somewhere in size between a sparrow and a blackbird, to declare naval warfare on four adults in a 60-foot, 20-ton narrow boat.
The instinct to protect the next generation and therefore your own genetic inheritance is really strong in all living things. It is always at its most powerful in spring when birds and animals are busy bringing the next generation into the world.
Kingfishers are doing well in Britain at the moment - populations have doubled in the last few years.
Canals and lakes are are getting cleaner. Kingfishers need this clean water in which to hunt tadpoles, small fish and aquatic insects.
A kingfisher needs to eat its body weight in fish and insects each day.
With chicks to feed, like our canalside neighbours, a breeding pair must hunt during every moment of daylight.
Typically there will be seven chicks in the unlined burrow and perhaps two or three clutches of chicks each year. That means the adults must catch about 5,000 fish throughout the summer.
The Royal Society for the Protection of Birds (RSPB) estimates there are between five and eight thousand kingfisher pairs breeding across Great Britain and Northern Ireland.
Kingfishers are protected under Schedule 1 of the Wildlife and Countryside Act 1981. It is illegal to take, kill or injure a kingfisher or its nest, eggs or young, or to intentionally disturb the birds during breeding season.
Sounds like that might include mooring a boat in front of their nest hole.
In the past, the little birds have been a victim of their own beauty. Their electric blue plumage was sought after in Victorian times. Feathers, wings, even whole birds adorned the hats, stoles and dresses of society women.
Taxidermists could sell as many stuffed kingfishers as they could catch.
I prefer my kingfishers alive on the canal bank - even if they do have to tell me to get out of the bloody way.
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