The latest report from Michael Walter:
Snow is never going to be good news for birds, but the freezing rain on 1st March may have spelt the death-knell for many. Under abnormal conditions a warmer layer of air can develop some height above ground; snow falling through it melts to rain, which instantly turns to ice on impact with ground that is below freezing (when this happens on roads it is referred to as lethal “black ice”). This freezing rain coats everything it hits – every leaf and every twig – with a thin layer of ice, locking away any insect food that may be beneath it; in contrast, a snow covering may drop off or be blown away, so providing small birds with some limited feeding opportunities. Freezing rain is therefore extremely difficult for small insectivores like wrens to cope with.
The graph above looks rather complicated, but essentially shows two things: the number of wrens breeding in one area of Blean Woods each spring (the dotted line, with scale on the left), and the mean temperature of the preceding November to February period (vertical bars, with the scale, in degrees Celsius, on the right). If you look closely you will see that the two measurements are correlated: when the winter is mild (tall bars) the wren population the following spring tends to be higher (peaks in the dotted line). Conversely, in cold winters (short bars) the wren population dips (troughs in dotted lines). In other words, hard winters kill off a lot of wrens, and the evidence so far is that there are few of them in the wood this spring. Small birds have a higher ratio of surface area to body mass than larger species, and so lose heat more quickly. They therefore need to eat more voraciously than larger birds in order to maintain their body temperature, but the wren’s diet is restricted to insects, which are in short supply, even in a mild winter, so freezing rain compounds the problems experienced in cold winters, putting these tiny birds in real difficulty.
Having highlighted the hawfinch in my last article, and said that I had failed to find any in the wood this winter, despite the autumn influx of continental birds, I finally heard one call twice early one morning recently. However, it wasn’t in typical hawfinch habitat, and may simply have been a foreign bird making its way back to eastern Europe, so I am not getting my hopes up yet in assuming this indicates an imminent recolonization of Blean Woods. More records would be needed later in the spring for me to be certain that this elusive bird is indeed making a comeback.
Michael Walter
michaelwalter434@gmail.com
01227 462491
A wren, one of our smallest birds, which may have suffered heavy losses in the recent cold weather.
Photo courtesy of Dave Smith