Garden Birds - Mike Toms Collins New Naturalist Library

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control group males, but no difference was found in estradiol levels in the females. Estradiol mediates maturation of the ovarian follicles and also appears to stimulate nest building and solicitation behaviour, so the researchers had hypothesised that there would be differences in this key hormone between groups. That the high-protein diet influenced a key male hormone rather than a key female hormone could suggest that male scrub-jays play a larger role in determining when reproduction commences than the females. The corticosterone findings were particularly interesting because they revealed that the levels of this ‘stress’-related hormone were lower in suburban populations of the scrub-jays than in either the HFHP, HFHP or control group – though HFHP birds had lower corticosterone levels than HFLP or control group birds. This suggests that the predictability of food resources may influence corticosterone levels and that corticosterone may play a role in the timing of reproduction. This work highlights the complex pathways by which something as apparently simple as the provision of additional food resources may have on the timing of breeding in birds.

      Work on species found breeding in UK gardens has generally identified a similar pattern, with the provision of supplementary food leading to earlier laying in Blue Tits in Northern Ireland (Robb et al., 2008a). Robb’s Blue Tits laid an average of 2.5 days earlier than unfed birds. However, Plummer et al. (2013), working on Blue Tits, found that laying dates did not differ significantly between their fed and unfed groups. It is worth just noting that most of these studies have been carried out in woodland habitats rather than in gardens, something that reflects the very great difficulty in running controlled experiments within an urban setting (see Chapter 6). Elsewhere, the laying dates of Willow Tit and Crested Tit were advanced by 2–5 and 5–8 days respectively when provided with sunflower seed and a tallow-based mix containing soy protein, wheat germ, sunflower seed, vitamins and minerals (Brömssen & Jansson, 1980). In a review of 59 studies, Robb et al. (2008b) found that 58 per cent reported significantly earlier laying dates in supplemented populations. Some studies, but not others, suggest that supplementary food may shorten incubation periods (Nilsson & Smith, 1988; Nager et al., 1997, Harrison et al., 2010). The Harrison et al. (2010) study revealed that supplemented Great Tits reduced their incubation period in each of the three study years, while Blue Tits did so in two of the three years.

      FIG 34. The presence of supplementary food at garden feeding stations can advance the timing of breeding in species like Blue Tit, something that has been proved experimentally here in the UK. (Jill Pakenham)

      Clutch size, egg and offspring quality and fledging success

      The formation of an egg is not only costly in terms of energy but also in terms of nutrients, such as protein. If females are limited by resources as they go into the egg laying period, then it follows that the provision of suitable resources may lead to an increase in clutch size. However, the evidence for this is mixed, with some authors finding an effect (e.g. Soler & Soler, 1996) but not others (Brömssen & Jansson, 1980; Svensson & Nilsson, 1995). In one case, the provision of supplementary food appeared to reduce clutch size significantly in Blue Tits and Great Tits breeding in a Worcestershire woodland (Harrison et al., 2010). The Harrison study revealed an interesting mix of results; in addition to the significant reduction in clutch size and the shortened incubation periods noted above, food supplementation from several weeks pre-laying through to hatching appeared to reduce hatching success in Blue Tits, but not Great Tits. Robb et al.’s review, mentioned earlier, found that in 44 studies that dealt with food supplementation and both laying dates and clutch size, 12 reported increases in both and 16 reported increases in clutch size alone (Robb et al., 2008b).

      FIG 35. Supplementary food has been shown to influence the number and quality of offspring produced by birds like Great Tit. (Mike Toms)

      The provision of supplementary food, just prior to and during egg laying, has been shown to result in increased deposition of carotenoids within the yolk of an egg, leading to increased hatching success (Møller et al., 2008) and, subsequently, higher rates of adult survival (McGraw et al., 2005). Chicks hatched from eggs with high levels of antioxidants have been shown to have lower rates of mortality in the days following hatching (Royle et al., 2011), so micronutrients stored by the female bird and derived from supplementary food, could deliver benefits to the resulting chicks. However, the findings of various studies show some differences. Supplementary feeding during the winter months – this time with fat – has been shown to affect investment in egg production, resulting in a smaller relative yolk mass in larger eggs and reduced yolk carotenoid concentration in early breeding Blue Tits (Plummer et al., 2013); these negative effects were absent in Blue Tits fed with fat plus vitamin E (see below). Egg size increases were reported in 38 per cent of the studies examined by Robb et al. (2008b). Returning to Florida Scrub-jays, Jim Reynolds (Reynolds et al., 2003) found that females fed with a high-fat high-protein diet produced heavier third eggs, containing more water and more protein than the eggs of unsupplemented individuals.

      Of course, one of the key figures when discussing breeding success is the number of chicks that fledge from the nest; while this is obviously influenced by some of the factors we have just discussed – such as the number of eggs – it is worth looking at whether supplementary feeding influences this figure. Working on a Blue Tit population in Northern Ireland, Gillian Robb and colleagues found that supplementary feeding significantly affected the numbers of chicks that fledged, such that individuals for which peanuts had been provided during the winter months fledged almost one extra chick per nest compared to those without access to the food (Robb et al., 2008a). Although Brömsson & Jansson (1980) did not find any effects of food supplementation on clutch size or the number of fledglings in Willow Tit or Crested Tit, they did find that Crested Tit nestlings were significantly heavier within the fed study site and that there was a tendency to produce second clutches more often in the experimental area.

      One area where concern has been raised about the practice of feeding garden birds during the breeding season is in relation to nestling health and well-being. It has been suggested that garden birds might feed their nestlings on artificial foods provided at garden feeding stations in preference to the natural, predominantly invertebrate-based, foods that they actually need. This suggests that garden birds are either lazy or unable to identify foods as being unsuitable for their nestlings. However, given what we know about food selection behaviour in wild birds, and which has been presented elsewhere in this chapter, both of these suggestions seem unlikely. Cowie & Hinsley (1988a; 1988b) examined nestling diet in a population of suburban Blue Tits and Great Tits, using nest cameras to identify the food provided by parent birds and how this was delivered. The cameras revealed that, on average, 15 per cent of the nestling diet of each of the two species was made up of artificial foods, notably bread, fat and peanuts. The amount provided varied between pairs, ranging from 6 per cent to 34 per cent in Great Tit and from 6 to 28 per cent in Blue Tit. Despite this, the researchers failed to find any relationship between the amount of artificial food provided and measures of nestling health. That adult garden birds are able to make appropriate decisions about the food they provide to their chicks can be seen from observations that if additional insect food is made available to nesting tits, or if their brood size is artificially reduced by removing some of the nestlings, the parent birds respond by dropping artificial foodstuffs from their diet and increasing the numbers of invertebrates provisioned.

      FIG 36. Adult birds appear to be able to identify appropriate foods for their nestlings and fledglings, with most species feeding their young on invertebrate prey while they are still in the nest, and making use of bird table fare as the young develop towards independence. (John Harding)

      Supplementary food, such as that presented to birds within the garden environment, may have consequences for other measures of breeding success, and indeed for other aspects of the breeding cycle. Studies have revealed that nesting adults, provided with additional food, will often reduce foraging effort, something that can provide time for other activities – such as predator vigilance

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