January was a brutal month. It brought storms, snow, ice and, when the thaw came, a turbulent River Wear. Finding food must have been a struggle for the heron that we’re watching through the riverbank alders today. The river is tranquil, but this is a tense moment: the final stage in catching a fish.
Motionless, it stares into the water, then oh-so-slowly extends that long neck. That dagger bill edges closer to the surface until, faster than the eye can follow, a splash. Success. It’s gripping something small and wriggling; a minnow or a miller’s thumb perhaps. At this time of year, most larger fish are downriver in deeper water.
Herons’ winter survival depends on far-ranging foraging and an adaptable diet: earthworms from waterlogged pastures, small rodents in grassland and even raids on suburban garden ponds in spring, when courting frogs provide all-you-can-eat opportunities. But through the coldest months, fishing provides their best food source.
Acquiring the necessary skills must be a steep learning curve. How do they avoid being bamboozled by refraction – that phenomenon where light travels slower through water than through air, so straight sticks appear bent when you poke them into a pond? Underwater objects appear closer than they really are, so surely herons need to learn to make allowances. To hit, they must aim to miss, though by how much depends on the depth of the water.
Compensating for refraction isn’t the only problem they face. Surface reflections must be confusing. Turbid water after heavy rain will cloud their vision. And predator and prey can see each other. Refraction is minimal when the bird’s line of sight is directly above its underwater prey, but that’s when it’s most likely to be spotted by a wary fish. So maybe the best compromise is that shallow angle of approach, delaying that downward strike as long as possible. Or is that an evolutionary just-so story explaining why herons need long necks?
We almost felt like giving our young heron a round of applause. About one in four juveniles make it through a second winter to reach breeding age. Mastering that compromise between a slow, low approach and a delayed strike must have been critical for its survival.
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount
