frosts affect wide areas irrespective of the number of animals present which must all suffer in a density-independent manner. A measure of density-dependence may be imposed if the animals are able to compete for restricted pockets of food; but this is a special case.
In south-west Europe, Cetti’s warbler is one of a small group of resident warblers which are similarly sensitive to hard winters and this applies to the resident Dartford warbler which is on the edge of its range in southern Britain. Cetti’s warbler spread north in France during the 1940s and 1950s with the period of mild winters and these also enabled a large proportion of Dartford warblers to survive in winter and then spread to other suitable habitats; a similar population increase and irruption to new areas occurred in the bearded tit for the same reason. The Dartford warbler formerly extended from Suffolk and Kent to Cornwall, but fragmentation of suitable heathland at the turn of the century has virtually restricted it to the New Forest, this being the only large enough area of suitable habitat, within its range, which can provide stability. Tubbs has demonstrated how during open winters the population can build up to occupy other heathland in Surrey and north Hampshire, where too small a population exists to withstand bad winters and where it has been exterminated two or three times since the 1940s – the winter of 1947 proving particularly disastrous. Tubbs has rightly emphasised the importance of maintaining the New Forest as a reservoir for the species. Here its numbers increased from around 80 pairs in 1955 to 382 pairs in 1961. Then came the snow of 1962 which exterminated the species in Surrey (the birds were trapped while roosting in tall heather by an overnight blanketing fall of snow) and left only 60 pairs in Hampshire. The following hard winter of 1962–3 virtually exterminated even the New Forest population, as well as causing a big retreat to the south of Cetti’s warbler in France.
Apart from changes in range caused by weather, several species have recently increased in response to addition to available habitats, which are often man-made. In inland Eurasia the little ringed plover replaces the ringed plover and nests on the sand and pebbly shores of slow-moving rivers or inland lakes, predominantly near fresh water. A pair nested at the Tring reservoirs, Hertfordshire, in 1938-the first nesting record in Britain. No further nests were found until 1944, when two pairs nested in another part of Hertfordshire, and one pair in Middlesex; the increase since then is shown in Fig. 6. By 1962 there were approximately 157 in Britain, nearly two-thirds being concentrated in the counties south of the Welland-Severn line and none extending further north than Yorkshire or west of Gloucestershire or Cheshire. Parrinder, who has documented these changes, points out that gravel pits provide over three-quarters of the nesting sites of little ringed plovers; sewage farms, reservoirs, brick pits and such places comprise the remainder. Most gravel-pits are centred in the areas where the species already occurs, except that a surplus of potential sites appears to exist in Lancashire, Northumberland and Durham and parts of Wales and these sites may next be occupied. Parrinder emphasises how gravel and sand production for building and road construction have increased with the post-war building boom and, as much of the material is derived from new workings, these have also increased. There can be little doubt that the creation of a new environment has favoured the species. It is interesting that our ringed plover has not been able to colonise these places, particularly as a local inland breeding population occurs in the East Anglian Breckland and the bird also breeds inland on some of the east Suffolk heathlands and in parts of northeast Scotland, for example in the Abernethy Forest.
FIG. 6. Increase in number of pairs of little ringed plovers summering in Britain. (Data from Parrinder 1964).
FIG. 7. Changing status of black redstart in Britain showing increase in number of territory holding males during the 1939–45 war with a decline following the final clear-up of war damaged sites after 1950. The solid line gives figures for the whole of Great Britain, while the dotted line is the contribution made by the City of London and Dover combined. (Data from Fitter 1965).
The black redstart was originally a bird of the warm montane regions of the southern Palaearctic, like the rock thrush and crag martin, but spread northwards after the last glaciation. It has adapted itself to a man-made environment, using buildings for nesting sites in lieu of cliff faces, and in parts of Germany it replaces the robin as the familiar follower of man. Its northward spread was still in progress across Germany during the last hundred years, and it only reached Jutland in the second half of the nineteenth century, and Scandinavia in the early 1940s. There is no record of it in Britain before 1819, though it became a regular passage migrant on the south and east coasts during the middle of the nineteenth century. Sporadic nesting attempts followed, in Durham in 1845, Sussex in 1909 and then a slow build-up from 1923 onwards. Three pairs nested at the Palace of Engineering in Wembley, Middlesex from 1926–41 but altogether less than half a dozen pairs were breeding in Britain up to the mid 1930s after which a big increase occurred, detailed in Fig. 7. The ability of the species to establish itself in this way has followed the sudden availability of nesting sites and feeding grounds, in the form of war-time bombed sites, particularly in London and Dover. Rebuilding and the post-war clean-up account for the subsequent decline in numbers.
There are also examples of bird numbers reduced through direct persecution, especially when the victim is fairly rare. The great crested grebe was certainly widely distributed in suitable places in Britain in the early nineteenth century, but by the middle years of the century a big demand arose for its breast feathers to make ‘grebe furs’ for a fashionable home market, and the slaughter began in 1857. By 1860, the species was reduced to 42 known pairs and was only saved by the sanctuary afforded by private estates, while further help came with the Bird Protection Acts of 1870–1880. Nevertheless, some increase was under way by 1880, before protection could have been very effective, and Harrison and Hollom (1932), who record these early changes, consider that human persecution came at the start of a period of long-term cyclical increase. By 1931, there were around 1,150 breeding pairs (with non-breeders, about 2,650 adults) in England and Wales and about another 80 pairs in Scotland. A sample census by Hollom (1959) showed that about the same number of adult grebes existed in Britain in the 1940s, but that an increase then began. When Prestt and Mills undertook a census in 1965 there were approximately 4,500 adults in Britain. This increase seems to have been favoured by man’s activities in creating numerous new reservoirs and gravel pits, just as the little ringed plover benefited. The 70% increase of the population in about twenty years can be compared with an increase of 84% in sand and gravel production between 1948 and 1957. That an increase followed the creation of new habitats also indicates that saturation had previously been attained and that the bird was regulated in the sense already discussed.
For a species to extend its range and take advantage of newly developed habitats it would be helpful for it to possess some kind of exploratory behaviour rather than rely on chance movements. It is becoming clear that immediately after breeding, many species, which normally migrate south, first indulge in northerly flights. The large-scale ringing of sand martins has shown that birds breeding at a colony in the south of England may move north to have their second brood, and juveniles marked in southern England have been found again in roosts in the north in the same season. Wood-pigeons also display northerly flights in September and October, before adopting a southerly orientation later in the autumn. Collared doves ringed in Europe as nestlings have moved north to Britain in the same autumn, and numbers of serins have turned up in south-west England in recent autumns. These movements seem adaptive in that young individuals which are surplus to the needs of the area in which they are born will stand more chance of finding new places to settle if they first explore north. The same principle applies to those birds of southeast and east Europe which might be expected to move north-west or west. I suspect that this factor may account for big arrivals of redbreasted flycatchers, woodchat shrikes, barred warblers, melodious and icterine warblers – all predominantly juvenile – into Britain in September 1958 and in subsequent years. Williamson showed that red-breasted flycatchers and icterine warblers arrived in Britain in clear anticyclonic weather with light winds, and as both migrate south-east to Asia, their movement several hundreds of miles
