Garden Birds. Mike Toms

      FIG 41. When supplementary food is available, both Blue Tits and Great Tits are less likely to join the mobile foraging flocks that are a feature of the early winter in woodland habitats in the UK. (Jill Pakenham)

      A feature of the tit species that are familiar to garden feeding stations here in the UK is their willingness to form mixed-species flocks during the autumn and winter months, a behaviour that also occurs in other tit species elsewhere in the world. This behaviour is thought to improve an individual’s chances of finding food during those periods when food is scarce and encounter rates low. You might, therefore, expect to see the provision of supplementary food exert an influence on the tendency to form or join mixed-species flocks. This is exactly what Thomas Grubb found, working on tits in deciduous woodland in Oxfordshire (Grubb, 1987). Grubb was interested in whether mixed-species flocking was driven by predation risk, by food availability or by both. His results suggested that food was the primary driver; Blue Tits and Great Tits without access to supplementary food flocked with other species while foraging more often than was the case when supplementary food was available. Interestingly, Long-tailed Tits – which ignored the artificial food – foraged in mixed-species flocks regardless of whether or not provisioned food was available. Work in Japan on Varied Tit Sittiparus varius underlines the behavioural flexibility in whether or not individuals join mixed-species flocks (Kubuta & Nakamura, 2000), revealing that individuals participate in mixed-species flocks to obtain the short-term benefits of increased foraging efficiency but, independent of food provision, they also obtain long-term benefits from the stability of their pair bonds and strong site fidelity.

      The studies just mentioned are relevant to the discussion of how the winter provision of food at garden feeding stations may alter the behaviour of the birds that visit. However, garden feeding may also change behaviour during the breeding season, something that may be of particular importance. As we shall see in our examination of the breeding ecology of garden birds (Chapter 3), the dawn chorus provides both a means to demonstrate ownership of a breeding territory and a mechanism to advertise your status as a mate. Research suggests that singing at dusk and, particularly, dawn may provide an honest signal of the energetic status of the singing male. If this is the case, then we might expect the availability of food in garden feeders to shape the performance of those males with access to it. Experimental work on Blackbirds supports this, with Cuthill & MacDonald (1990) finding that food-supplemented males sang significantly more than unsupplemented males. This difference was largely the result of supplemented males initiating song earlier and having higher peak rates of song delivery. Could this mean that male Blackbirds with access to a reliable food source at garden feeding stations are more likely to attract a mate than those nesting in other habitats where perhaps food is more limiting? A similar piece of work, this time on Great Tits, casts some uncertainty on whether the relationship is as simple as it appears from the work done on Blackbirds. Katja Saggese and her collaborators provided male Great Tits with a continuous food supply over two weeks and then compared their singing activity with a group of unfed males (Saggese et al., 2011). In contrast to Cuthill & MacDonald’s findings, the food supplemented males started their dawn singing later than the control males, an effect that still continued two weeks after the provision of food had ended. The researchers were unsure of the reasons for what they had observed, but they felt that it could have been due to the presence of predators, attracted to the feeding stations, or something about the quality of the food itself.

      FIG 42. Male Blackbirds with access to supplementary food sing for longer than those without access to such food, suggesting that the food provided at garden feeding stations might help local birds in defence of their territories and mate attraction. (Mike Toms)

      FOOD PROVISION AND MOVEMENTS

      Birds tend to reduce the size of the area over which they forage when provisioned with supplementary food, something that can lead to a reduction in the size of breeding territory and even a change in mating system in the case of Dunnock (Davies & Lundberg, 1984). As we will see in a moment in relation to Blackcaps, the provision of food in gardens can have a significant impact on movement and wintering behaviour.

      Although not garden-related, the scale of feeding impacts can be seen from work on the White Stork Ciconia ciconia, a species that was wholly migratory in Europe but which has established resident populations across parts of Iberia largely in response to the year-round food available at landfill sites (Gilbert et al., 2016). Landfills and food discarded by people have also played a role in the expansion of urban-breeding Lesser Black-backed Gulls Larus fuscus here in the UK (Coulson & Coulson, 2008), a species that now winters here in large numbers, when it formerly wintered in southern Europe and north and west Africa.

      FOOD PROVISION AND THE EVOLUTION OF NEW BEHAVIOUR – WINTERING BLACKCAPS

      The Blackcap is a common summer visitor to much of Britain, wintering in southern Europe and south into North Africa. Although there are occasional records, the Blackcap was rarely encountered in Britain during the winter months 60 years ago (Stafford, 1956). However, since the 1950s we have seen a rapid and substantial increase in wintering records, as information from both the Garden Bird Feeding Survey and Bird Atlas 2007–11 show (Balmer et al., 2013). Data from BTO-led bird atlases show that the Blackcap’s wintering range in the UK has expanded by 77 per cent over the last 30 years. The increase in wintering numbers has come about because Blackcaps breeding in southern Germany and Austria have increasingly migrated in a northwesterly direction to Britain for the winter, rather than in a southwesterly direction to traditional wintering areas located in southern Spain (Helbig et al., 1994). This new migration strategy has been shown to be genetically encoded (Berthold et al., 1992), and is maintained via reproductive isolation and assortative mating, linked to fitness benefits on the breeding grounds (Bearhop et al., 2005).

      FIG 43. The use of gardens by Blackcaps peaks during the winter months. Data reproduced from BTO Garden BirdWatch with permission.

      Early observations of increasing numbers of wintering Blackcaps coincided with the introduction of commercial bird foods (Callahan, 2014), suggesting that supplementary feeding at garden feeding stations might be applying a selection pressure for the evolution of this new migratory route. Interestingly, while the Blackcaps wintering in southern Spain are predominantly frugivorous in their dietary choices, those wintering in Britain are known to use a wide variety of supplementary food types (Tellería et al., 2013; Plummer et al., 2015). The availability of supplementary food is unlikely to be the sole driver of evolutionary change, however, and it is likely that a changing climate has also had some influence over the adoption of this new migration route and new wintering area. Winter conditions in Britain, and more widely across the Northern Hemisphere, are becoming milder, something that has enabled a number of species to shift their wintering range northwards (IPCC, 2013).

      Unravelling these different drivers requires access to long-term datasets on food provision, Blackcap occurrence and climatic factors. Fortunately, BTO researchers have been able to use data from BTO’s weekly Garden BirdWatch survey to explore the question of whether food provision has played a part in the evolution of a new migratory strategy in this central European Blackcap population (Plummer et al., 2015). Earlier work using the BTO Garden BirdWatch dataset has revealed that Blackcaps are strongly associated with suburban gardens during the winter months (Chamberlain et al., 2004a) which makes this dataset ideal

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