Action: Provide supplementary food
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A replicated, controlled study from Europe found that overall, gardens with supplementary food did not contain more species than those without. However, there was some evidence that gardens with supplementary food in five countries did contain more species than unfed ones, when countries were analysed separately.
For specific interventions see: provide supplementary food
Food supply is one of the key factors determining mortality and reproductive rates. Providing supplementary food is therefore often used as a technique to support small populations. However, feeding is only likely to have a positive effect on a population if the food supply is limiting either reproduction or survival. Because of differences in population responses to food during the breeding season and at other times, we have divided studies into those investigating the impact of feeding on reproduction and those investigating adult survival.
As with all interventions in this synopsis, studies that investigate population-level effects are most useful for conservationists. This is especially true for supplementary feeding, as many birds have large foraging ranges and the appearance of increased numbers at a feeding station, or even in the habitat surrounding feeders may not represent an increase in numbers but a redistribution of the same birds, and could even hide a population decline.
It is also important to note that the effect of providing food can be confounded by many factors. For example, variations in natural food supplies due to population cycles or irregular fruiting, whilst droughts or other extreme weather and pollution levels can also affect how populations respond to food.
Providing supplementary food can also be used to improve the success of release programs for captive-bred birds. Studies describing this intervention are discussed in ‘Captive breeding, rearing and releases (ex situ conservation) – Provide supplementary food after release’.
Supporting evidence from individual studies
A replicated controlled study of 440 gardens across 14 countries in Western Europe (excluding the UK) from October 1988 until May 1989 (Thompson et al. 1993) found that 264 gardens frequently provided with supplementary food were visited by an average of 21 species of birds, compared with 22 species for 40 moderately-fed gardens and 21 species for 116 infrequently-fed gardens. Differences were not significant. There was considerable variation across the study area, and feeding frequency appeared to affect the number of species visiting gardens in France and Switzerland, with 17.5 species visiting four infrequently-fed gardens, 13.3 species visiting four moderately-fed gardens and 20.7 species visiting 104 frequently-fed gardens. There was a similar, weaker effect for West Germany, the Netherlands and Belgium. Frequently-fed gardens were provided with food in more than two-thirds of the weeks studied, moderately-fed ones were provided with food for between one and two thirds of the weeks and infrequently-fed ones were provided with food in fewer than one third of the weeks studied.