Several participants of the House Sparrow Project (HSP), also participated in the Garden-Bird Survey (GBS). This prompted me as co-ordinator of the HSP to have a look at the data gathered on House Sparrows in the GBS survey. At first I was rather reluctant to do so as GBS counts consist of weekly maximum numbers of each bird species observed in a particular garden. In the GBS there are no limitations to the number and duration of the observations per week and as participants are free to provide food, it is unclear if the data produced by a participant in both surveys could be mutually comparable throughout the year, and even less clear if data collected by different participants could be mutually comparable and of use for analysis.
HSP participants are required to determine maximum numbers of House Sparrows for 14 days in spring and autumn by counting for 30 or 45 minutes daily after the distribution of a measure of mixed bird seed. On account of the differences between the survey methodologies it was clear that HSP and GBS data could not be treated in the same way and together these considerations gave rise to my initial sceptical attitude to making these comparisons.
However, a little later it occurred to me that the GBS data were important since they could provide a picture of variations in the size of the local House Sparrow population over the course of a year and perhaps could also be useful in determining the best time for the spring and autumn counts. The idea behind the spring count was to determine the size of the breeding population before the start of the breeding season, whereas the autumn count was meant to provide an impression of the success of the breeding season. The timing of both counts therefore was arranged to serve both these purposes, and most importantly, the autumn count could be expected to reflect the post-breeding peak population which would include the young birds of that year.
Still later it occurred to me that there were even more special aspects to the GBS data that could be of use in the interpretation of HSP data. In particular, within the GBS, participants not only count House Sparrows but also record all other species which show up in their gardens. This introduces the possibility of relating changes in House Sparrow densities to the presence or absence of other birds, and especially to predatory birds such as Magpies and Sparrowhawks. Together this could lead to interesting correlations.
The local bird club provided access to the GBS data collected in 1997 for analysis, covering the region of Gooi and Vecht in the Netherlands. In order to reduce compatibility problems, as much as possible, only data of GBS participants who counted in all 52 weeks of the year was used. A total of eighteen participants met this requirement.
Sparrow-poor gardens were separated from sparrow-rich gardens based on weekly counts of below and above a record of 3 House Sparrows in each garden surveyed. Adherence to this simple definition resulted in data for 9 sparrow-poor and 9 sparrow-rich gardens eventually being analysed.
The participating gardens were more or less arbitrarily spread across the built-up areas of the Gooi & Vecht region. The distance between gardens was consistently greater than 250 meters and the majority exceeded that distance many times. Since the home range of House Sparrows is only a few hundred meters it was assumed that each garden would act as an independent sample. In other words, it was assumed that no participant could count the same birds.
Since the HSP had shown that the provision of food strongly influenced the number of birds observed, this was the first aspect of the data that was analysed. GBS participants are required to indicate on their forms whether they supply food on the ground, at a feeding station or hanging in bushes or trees and whether it is done so on a regular basis, only sometimes, or not at all.
In order to be able to process this information in a quantitative way, 2 points were given for a "regular food supply"; "occasional food supply" 1 point and "no feeding whatsoever" 0 points. Therefore, each garden could get between 0 and 2 points in respect of the three ways food was supplied and therefore could score between 0 and 6 points per season. These points were apportioned in respect of the 9 sparrow-poor and the 9 sparrow-rich gardens, and the added points were plotted as "feeding intensity" in Fig. 1.
Fig. 1 has shown that the feeding intensity was comparatively low in spring and summer in both types of gardens. This was because it was found that in sparrow-poor gardens more food was supplied in winter than in sparrow-rich gardens, whereas it was the other way around in the autumn.
Next, a graph was constructed for both types of gardens, depicting the changes in the numbers of House Sparrows and Magpies present over the year (see Fig. 2). In sparrow-rich gardens (Fig. 2A) a weekly maximum of approximately 10 House Sparrows was recorded in winter. The weekly maximum slightly decreased on the approach of spring (April-May), and from June onwards a steady increase was found to set in.
At the end of the summer and the beginning of the autumn the population of House Sparrows is at its yearly peak, after which a slight decrease returns. The decrease in spring may have several causes. Firstly, there may be decreased feeding intensity, and secondly, incidence of decreased visibility by budding leaves and an absence of observations of adults because of reproductive duties elsewhere (sitting on eggs!), all seem to play a role. In point of fact it is amazing that the decrease is so small.
The increase from June onwards sets in too early to be explained by an increase in feeding intensity and is without doubt a consequence of the appearance of young birds some of which probably depart in the autumn.
The seasonal variation in sparrow numbers in both types of garden in response to the provision of food is not found in Magpies. The Magpie pair defends a territory during their breeding season and it is therefore understandable that rarely more than two Magpies were found simultaneously in any one garden. Whenever it did happen, it concerned either parents with their young or in rare cases a so-called "gathering". The average weekly maximum number of Magpies was found to be nearly constant throughout the year and fluctuated around the value of 1. By definition there were few House Sparrows in sparrow-poor gardens (See Fig. 2B): weekly maxima fluctuated between 0 and 2 without any significant seasonal pattern. Sparrow-poor and sparrow-rich gardens did not differ in the numbers of visiting Magpies (Fig. 2A vs. 2B).
The counting periods laid down for the HSP (a fortnight in spring between 1 March and 15 April and a fortnight in the autumn between 15 September and 31 October) appear fairly well chosen. It seems reasonable to assume that the entire stock of breeding adults would be included in the spring counts between the chosen dates. However, the autumn counting period might benefit from an earlier date as a greater number of young birds might be included if it were held in a count period starting on 15 August. And in conclusion, as sparrow-poor and sparrow-rich gardens were visited by Magpies at similar frequencies, it would seem to be extremely unlikely that these predatory birds play a dominant role in the regulation and in the overall decrease in the numbers of House Sparrows around the urban residential areas of the Gooi & Vecht region.
Created on ... July 10, 2000
Revised on ... August 11, 2000