Introduction
Recently the "Vogelwerkgroep 't Gooi e.o." our local birding club in the Netherlands declared the House Sparrow "bird of the year 1998". Newspapers, local and regional radio stations and even a nation-wide TV station helped us to bring the dilemma of the House Sparrow to the attention of the public. The message was simple, that of the startling decline of a once widespread species. Not surprisingly this subject attracted great attention and concern across the country as the possible reasons for this population decline was discussed.
A total of 66 people sent letters to the club and some contacted me personally by telephone to express their grave concern. Most people (59 responses) in discussion provided an estimate of the abundance of House Sparrows in their own home districts. Their observations strangely ranged from 'very numerous' (30 or more Sparrows) to 'completely absent', and it appeared that the latter category was alarmingly large (27 out of 59 responses). A little more than half of the people who lived in the large urban areas of the Gooi region indicated that House Sparrows were completely absent around their houses (Naarden-Bussum: 8 out of 15 responses; Hilversum 10 out of 18 responses). In contrast, in smaller municipalities in some less populated areas only 7 of 28 people reported a complete absence of House Sparrows.
Methodology
It was crucially important naturally to evaluate the accuracy and reliability of these estimates. Fortunately it was possible to examine the information received and form a reasonable opinion. All 66 people who took the trouble to respond to the press reports, received an invitation to participate in the ongoing investigation into the distribution of House Sparrows in their region, and appropriate survey guidelines and data collection forms were sent to each. Twenty-four people sent their completed forms back after having thrown mixed grains on a fixed place in their garden and on fixed times over a period of 14 days and then counted the numbers of House Sparrows which gathered at the feeding point each day.
A number of participants (16 out of 24) gave an estimate of the number of House Sparrows in their residential areas before they started counting their garden birds. Seven participants (out of 16) counted 6 or more House Sparrows which confirmed that their previous estimates had been reasonably accurate: on average they had estimated 23.3 birds, whereas they actually counted a mean of 19.6! Although eight participants had stated that there were no House Sparrows around their house, four of them were surprised to find that sparrows visited regularly. Indeed, they counted maximum numbers of 6,8,10 and 13 Sparrows respectively.
A total absence of House Sparrows was declared by four other participants on their preliminary counting forms based on their counts of birds attracted to their gardens, such as Collared Doves, Blackbirds and several Finch species; but no House Sparrows were observed over the 14 days of the feeding cycle. Therefore, declarations by participants that "House Sparrows are no longer recorded in my garden and the local residential area" were viewed with great caution as in 50% of the cases it was subsequently shown that there were reasonable numbers of sparrows present, provided the right bait and the recommended method of observation had been followed. As the accuracy of these data were considered unreliable it was decided not to plot the estimated numbers of House Sparrows on the maps of Hilversum, Bussum and Naarden, although these cities would most likely have provided interesting and valid results in view of the number of responses which had been received from them (see Table 1).
Results
For the second consecutive year 18 loyal participants counted House Sparrows. They were joined this time by 24 new participants, who submitted 24 completed counting forms. These data formed a new, independent sample and followed the previous years results of the first group (see Paper 1) which had indicated, to the surprise of many experts, that the House Sparrow has become rather rare in the vicinity of older houses. I was therefore very interested to know if this new, somewhat larger sample would produce a similar picture.
The maximum number of House Sparrows which were counted were plotted against the year in which the houses were built (see Fig. 1). To my great satisfaction a positive relationship appeared. A regression analysis (a statistical test to determine if an observed relationship could be relied upon or whether it might have happened by chance alone) was applied to measure the positive relation between the number of Sparrows and the dates of the construction of the houses, to determine if it was statistically significant. In other words the older the houses, the greater the chance that there would be no Sparrows present.
The graph shows that the regression line does reflect a statistical relationship, but that it also represents the wide range of responses from the participants. For example there was one older house with many Sparrows around and conversely there was also one more recently constructed building without any Sparrows.
In summary, it can be concluded that the independent sample of 24 newly enlisted participants produced almost identical results to the original sample of 18 participants. These new results endorse the earlier conclusions that the House Sparrow had largely disappeared from the older residential areas, whereas birds were still found in high densities in the newer city districts and in the smaller, more rural municipalities.
Nesting opportunity
This poses the question of what might account for the variations found in the distribution of House Sparrows between the older and newer residential areas. In the absence of detailed long-term research it was considered appropriate to make a few educated guesses. My first guess was that the presence or absence of nesting opportunities was not a critical element. Most tenants of the older houses with low numbers or a complete absence of sparrows stated that historically House Sparrows were regular breeders there but that numbers had decreased rapidly over a number of years. These changes did not seem to be caused by building improvements with the closure of former nesting holes which deprived sparrows of nesting opportunities. In contrast, the absence of nesting opportunities owing to improved building practices might have been expected more frequently in the newer houses. However, it should be kept in mind that although the House Sparrow is a resident bird, it still has an action radius or home range of several hundred meters. Therefore changes to individual houses should hardly have had any detrimental effect, although measures taken on a district level might well have been expected to do so.
Predation
It is considered that the relatively low density of House Sparrows in the older residential areas cannot be explained by local avian predation. Magpies and Jackdaws are usually mentioned in this respect as they are known to take sparrow eggs, chicks and even adult birds. Cats also earn a bad reputation as they frequently prey both on the young and adult birds. Naturally some birds do fall victim to one or other of these predators, but the point is whether we consider them capable of the systematic eradication of their prey, and if so why it occurs only in the older residential areas. In any case, predatory birds and cats are present in nearly all residential areas and it is unlikely that these pressures could affect the distribution of the sparrow and account for the major collapse in the numbers of birds found in recent years.
Food availability
Although waste edible materials can be found in the streets of our cities and food is even regularly provided for birds in many gardens, I think that the general availability of food, suitable for seed-eating birds such as House Sparrows, is increasingly rare in most modern urban environments. However, the main difficulty seems to be the lack of insects in the spring and summer which are absolutely essential for raising nestlings and fledglings (Van der Plas-Haarsma, 1980). In view of their rather limited home range House Sparrows are therefore (perhaps increasingly) dependent on the proximity of rural areas with the associated abundance of suitable food, particularly in the breeding season. Because of the intense building activities over the last 25 years, especially around the older city centres of the larger urban concentrations, these environments have drifted too far away from the rural areas. The new suburban districts are, through the eyes of a House Sparrow, much more favourably located in respect of access to the countryside. Hardly surprising therefore that House Sparrows have come to prefer suburbia to their former environments in the centre of the larger cities.
Literature
Moniek van der Plas-Haarsma: De Huismus. Uitgeverij Het Spectrum (1980), 156 pp.
Created on ... July 10, 2000
Revised on ... August 11, 2000