Twenty-five years ago, however, catching a blackcap was a rare event.
Once, as a trainee ringer, I got to handle one. My hands shook with excitement and the bird escaped before I had put the ring on. My trainer was not amused at the fumbling messer who had let a celebrity bird go unringed. That I might never catch one again was a reasonable supposition in those days.
The blackcap is one of our larger warblers. Adult males are most elegantly attired, dressed in grey business suits, fading to white on the chin and belly. The black legs and feet match the dark eyes and the heads are crowned with black skull-caps, like the ones Jewish men wear. The female plumage and cap are brown. The ladies are pretty but they lack the sartorial elegance of their mates.
The bird’s behaviour is also a model of discretion. Blackcaps don’t go in for noisy chattering like sparrows, nor do they kowtow to gardeners the way robins do. When visiting suburban gardens, they tend to keep a low profile, staying under cover; many householders are unaware that they have blackcaps in their garden. Riverside scrub is frequented, but blackcaps prefer mature broad-leaved woods to nest in.
The song is distinctive. A jerky warble, low-pitched for such a small bird, is heard throughout Spring.
The lower the pitch of a sound, the farther it travels and songbirds tend to sing at the lowest frequencies that they can manage. However, small birds and musical instruments have correspondingly small voice boxes and the smaller the box the higher the pitch. Only blackbirds can produce really low-pitched songs; they are bigger than the other garden birds. That the blackcap can generate so mellow a warble is remarkable.
Many of our Irish songbird populations have declined in recent decades but blackcaps have prospered. Back in 1937, GR Humphreys, in the List of Irish Birds which he prepared for the National Museum, described the blackcap as a scarce summer visitor. It was “most frequent in Wicklow”, he claimed. Things had not changed much 30 years later; Robert Ruttledge, in Ireland’s Birds, published in 1966, noted that “the blackcap is extremely local in its distribution; Co Wicklow is its chief stronghold”.
However, within a decade, the situation was transformed; during the surveys for the Atlas of Breeding Birds in Britain and Ireland, carried out between 1968 and 1972, blackcaps were found throughout most of Leinster and parts of Munster and
Ulster. The Atlas project was repeated between 1988 and 1991. By then, blackcaps were breeding in every Irish county. The number of 10km squares in which they were present had increased by 40% and, for the first time, an estimate of the total population was made. There were thought to be 40,000 breeding pairs in Ireland. Numbers are still increasing; according to the Countryside Birds Survey, published by BirdWatch Ireland, the population is growing at about 25% annually, the fastest increase of any Irish bird.
But what brought about such an extraordinary change? Thanks to the efforts of ringers, we think we can answer this question. The blackcap seems to have benefited from global warming, a conclusion which is evident from the ringing data. Over 400,000 blackcaps have been ringed in Britain and Ireland and their movement patterns are now fairly clear. They are most intriguing.
Almost all of our Irish, and indeed British, blackcaps leave these islands in the Autumn. According to the British Trust for Ornithology’s Migration Atlas, only one ringed blackcap, known to have bred in Britain, was found there subsequently in winter. Nor were any Irish breeding blackcaps found here. From the pattern of ring recoveries, it’s clear that the entire population moves to the south of France, Spain or Morocco. Some individuals go further, crossing the desert to sub-Saharan Africa. But if the local birds have gone, why are there so many blackcaps in Ireland in winter? The answer seems to be that these are foreign birds; blackcaps from mainland Europe.
All central European blackcaps migrate; conditions in winter are too severe for them to remain at home. German birds traditionally headed for the Mediterranean and some moved on across the deserts of North Africa. With the onset of global warming in recent decades, however, drought in the Sahel region has made the desert crossing longer and more hazardous. This seems to have hit migrating blackcaps and reduced their numbers. The direction in which a songbird moves at migration time is an inherited disposition. It seems that a minority of German birds have a propensity to travel westwards, bringing them to the Atlantic coast and to Britain and Ireland. With the milder winters of recent years, these birds have thrived.
Extraordinary as it might seem, our Irish breeding blackcaps birds leave each Autumn and their place is taken by birds from Europe.
Derek Langslow, writing in the Migration Atlas, suggests that this wintering pattern originated “in the last 30 years” but, in Ireland, it seems to have been underway somewhat earlier. In 1937, Humphreys noted some 50 blackcaps in Ireland in winter.