It begins on the path. The sunlight of a March morning cuts through the lobed leaves of the oak canopy. Steam flows from every breath; scarves wrapped tight against the cold. A trickle of muddy water flanks the path on one side, overhung by mossy boulders. On the other, the land angles down towards the valley floor, forested all the way.

      Before long, my friend Mark and I come to an aisle of skeletal birch and ash shrubs, stripped of leaves by a winter just ended. Families of long-tailed tits pirouette about the twigs above us, hoovering up any invertebrates stirred to life by the inklings of spring. Fresh buds provide the only greenery. They also make a ripe harvest for bullfinches, flitting through the shrubbery.

      We soon emerge at the valley floor. At its heart lies the ancient church of St. Saviour, roofless from centuries of neglect.

      The holy men who sought God in Glendalough could scarcely have picked a better spot for their church. Even a sceptic can admit that it commands a captivating view of creation. The brook that tinkles beside the church is hemmed in by hills. They are crested with conifers on one side and threadbare deciduous trees on the other. It’s as if each clan has staked its claim to either bank, with occasional copses breaking the trend, the vanguard of some horticultural crusade across the valley, forays onto enemy soil.

      The conifers are crawling with songbirds. Siskin, blue tit and chaffinch all poke their heads out from between the needles. Blackbirds and song thrush are in full verse. Every half hour or so, there’s the squawk of a pheasant. Further afield, the drumming of great spotted woodpeckers adds percussion to the ensemble. But the great tit champions them all with its relentless two-toned cry. Great tits are notorious bullies at bird feeders, and their competitive personalities also manifest in song. Long after the other birds have desisted, the gnawing teacher, teacher call still rings out across the valley.

      So much is patience with birdwatching. I think of sitting in hides overlooking reed beds or staking out valleys for raptors that fail to manifest. But sometimes nature is generous. A morning such as this was proof.

      The stream bisecting the valley flows right past St. Saviour’s Church, which is itself ringed by a mossy ridge. No sooner had we poked our heads over this, to gaze at the amber stream below, and there they were. Four goosanders in two handsome pairs, as if taken straight from the pages of a guidebook; the females with copper heads and silver backs, the males with their stunning white chest, black saddle and green head, darker still in the shadow of the trees. For both sexes, a blood red bill, drooping at the tip, completes the package.

      It is so quick I only have time to note their fine details before the flotilla, hurried but not panicked into flight, makes its way downstream, heads turned at 45-degree angles to keep us in view. The tangled undergrowth soon obscures them, and they vanish around a bend in the distance.

      …

      Goosanders are one of a brace of breeding ducks we have known as the sawbills (a third, the smew, is a scarce winter visitor). It is the largest of this family of ducks, which derive their name from the notched, fish-eating accoutrement they carry. Not for the sawbills is the clumsy, spoon-shaped beak of the dabbling ducks, the mallards that throng on urban waterways begging for bread from passers-by or the flocks that colour our wetlands each winter. Most of these birds, despite spending so much of their time on the water, can scarcely upend to crane for a seed beyond the reach of their outstretched neck.

      Not so the sawbills. Never content to languish at the surface, they get their food by diving. And far from subsisting on debris floating on the water’s surface, or grazing on waterside meadows like avian ungulates, they’ve embraced a predatory lifestyle, pursuing fish and invertebrates with singular agility.

      As with most birds that have taken to the water, their webbed feet are placed far back on the body for maximum propulsion underwater. (This, though, leaves them at a disadvantage on land, where they can only amble awkwardly.) And like most avian fishermen, they track down their prey by sight. In the case of the goosander, they frequently dip their heads beneath the water, scoping out the submerged surrounds for a meal. Once a fish has been spotted (amphibians and insects are also taken) the chase begins. If successful, the goosander usually surfaces with prey in its beak. Tenderly tossing it around in its saw bill, the meal is then swallowed headfirst, ensuring no spiny fins get caught and easing its passage down the bird’s throat.

      Fishing is when the saw bill comes in handy. Running up and down its length are tiny serrations, hooking backwards to secure tight purchase on slimy prey. In this way, the sawbills harken back to some of the most primitive birds, flightless behemoths who snapped up fish with toothed beaks while their dinosaur cousins still dominated on land. The goosander, though, is a bird of flight. Like all modern birds it has abandoned teeth entirely in order to shed the weight needed to take wing. The serrations, though, are about as close as any modern bird comes.

      The bill also has use during courtship. In the mating season, displaying males elongate their necks and bills skywards to their fullest extent, cutting circles in the water as they bid to woo passing females.

      The goosander shares its saw bill with its close cousin, the red-breasted merganser. At first glance, they appear similar. But there are important differences. Mergansers, for one, are primarily birds of our coasts, gathering in sheltered harbours by winter where they can fish in relative safety from the tumult of the open sea. Though attractive birds, their plumage is not clear cut; the colours merge and dilute, as if the bird has thrown on its cosmetics all in a hurry, only to have them blurred and sullied by the water.

      The goosander is a bird of wild lakes and rivers, only haunting the coasts in winter. And its sublime plumage (sans the windswept crest of the merganser) always retains its clear demarcations, especially in the male: that luscious green head atop a white nick and chest, flanked by darker wing markings.

      Preferring, as it does, forested habitats within touching distance of fresh water, the goosander has adopted a breeding strategy you’d think anathema for a duck, especially one so large (significantly bigger than most you’ll find at your local pond). It routinely nests in tree holes, a habit normally reserved for the much smaller songbirds with which it shares its woodland home. It’s as if the peculiarities of passerine-hood have rubbed off on the goosander, and so it endeavours to stake a claim to the most prized of nesting real estate the forest has to offer.

      Selecting a suitable nest site is the task of the female goosander. Her standards are exacting – and they have to be. Finding a tree hole large enough to house a family of goosanders is challenging enough. But the need to overshadow running water restricts the goosander’s nesting choices even further. It is onto this water (or, at the very least, a soft surface near the water) that young goosanders – still flightless – must crash when they leave the nest for the first time, or else risk a fatal fall straight onto solid ground.

      Most tree holes don’t meet the criteria: big enough to house the female and her brood at a squeeze, while being close enough to a stream or lakeside to allow a safe landing for the chicks. It’s not unheard of for hole-ridden trees to play host to several goosander families. Once she’s found a hole that satisfies her, the female fashions the bottom into a bowl, lining it with soft down plucked from her own breast. However, the chicks don’t get to enjoy this cosy bedding for long. Within forty-eight hours of hatching their mother’s call tempts them from the nest, out onto the water below.

      In their generosity, conservationists have erected nest boxes in Glendalough that the birds have readily taken to. In their absence, and if there’s a dearth of tree holes, goosanders are forced to compromise, making their homes under mossy boulders or even in the gutted ruins of homesteads. This means a walk over land for the female and her chicks to reach the lake or stream. Here, the youngsters are fed on aquatic invertebrates