Brittingham and Temple recognised that two different types of dependency may occur. Over the short term, birds visiting a feeder may come to expect that particular location to provide food; if the feeder is left empty or is removed, then the bird may suffer. Over the longer term, those individuals that are heavy users of feeders may lose the ability to recognise naturally occurring food items, lose the skills to handle them and suffer from reduced survival rates. Working on two resident populations of Black-capped Chickadee, Brittingham and Temple tested whether the population that had utilised feeders in the past experienced lower survival rates than a population that had never been exposed to them during a winter when feeders were not available to either population. The researchers found no difference between the average monthly survival rates of the two populations and concluded that there was no evidence for dependency in this species.
FIG 33. The question of whether wild birds become dependent on garden feeding stations has been largely answered by scientific studies; these indicate that small birds are aware of other feeding opportunities and will quickly turn to these if food is no longer available at a given site. (John Harding)
The question of whether the sudden cessation of food provision can have a negative effect on individual birds has not been tested but is not thought to be a particular issue. The thinking that supports this view comes from the work on chickadees, which demonstrates that individual birds track a number of food patches at the same time, sampling these no matter how abundant food is at any one patch (Brittingham & Temple, 1992b). For a chickadee living in the northern part of North America – and being one of the smallest bird species to remain so far north during the winter months – tracking a range of food patches has clear survival benefits; if conditions change and one patch is no longer available, individuals already know where other patches are located. The sudden loss of a feeding opportunity might be more of an issue for other species, but one would expect the urban environment and its gardens to provide multiple feeding opportunities within a short distance of each other. This is something that has been examined, to a small degree, through the work of Patrick Thompson, who looked at the effect of interrupting regular food provision in gardens on the numbers and behaviour of House Sparrows and Greenfinches (Thompson, 1989). Thompson’s work revealed that the daily pattern of feeding station use was unaffected by the number of neighbouring gardens providing food. However, the numbers of individuals using the experimental garden feeding station was dependent on food provision in neighbouring gardens. When the number of neighbouring gardens providing food was reduced, there was an increase in activity within the experimental garden, where feeding had continued.
THE VALUE OF GARDEN FEEDING STATIONS FOR WILD BIRDS
Many of the individuals providing food for birds at their garden feeding stations during the winter months do so because they believe that it will help the birds through a difficult period of the year. The provision of food might improve the survival rates of fed individuals, but to what degree. Work on Black-capped Chickadees in North America has revealed that supplementary feeding can make a significant difference to overwinter survival. The fed chickadees studied by Brittingham & Temple (1988a) were significantly heavier than unfed birds and, while this difference in weight was very small (equivalent to just 1 per cent of body weight and representing two hours of foraging time), it made a big difference to their longer-term survival rates. Fed individuals had a 70 per cent survival rate through to the end of the winter and beyond, compared with just 30 per cent in unfed birds. Work on Willow Tits and Crested Tits, carried out in coniferous woodland near Gothenburg in Sweden by Jansson et al. (1981), also found that the provision of additional food resource improved winter survival significantly. In this case, the three researchers noted that the provisioned food helped to reduce predation risk by allowing individuals to spend more time in far more protected places than they would frequent under natural food conditions.
FOOD PROVISION AND MEASURES OF REPRODUCTIVE SUCCESS
Various studies have demonstrated that increasing the availability of food resources through the provision of supplementary food can advance the onset of breeding, prolong the period over which breeding takes place and increase both the number of young raised per breeding attempt and the number of breeding attempts made. Work on Florida Scrub-jays Aphelocoma coerulescens (Fleischer et al., 2003; Schoech et al., 2004) has, for example, revealed that populations with access to supplementary food can start breeding up to 17 days earlier than populations without such access. Similar findings have been revealed for Australian Magpie (Rollinson & Jones, 2002).
The timing of breeding
The work on Florida Scrub-jays reveals both the differences that supplementary food provision can make to the timing of breeding and some of the mechanisms behind it. Suburban scrub-jay populations have access to supplemented food in the form of peanuts, cracked corn, birdseed, bread and pet food, with up to 30 per cent of their diet made up of such items (Fleischer et al., 2003). This food forms a predictable resource, which is in contrast to the unpredictable nature of resources experienced by wildland populations. Predictability appears to be important and probably explains why Fleischer et al. (2003) found their suburban scrub-jay populations to be consistent in the timing of their breeding across years, while wildland populations varied. Having a predictable food source in the form of supplemented food may well explain why suburban scrub-jay populations spend less time foraging and more time perching than wildland birds – scrub-jays use a sentinel system, with perching birds alerting other individuals to the presence of potential predators. Suburban scrub-jays also handled more food items per hour, suggesting that their foraging was more efficient. Interestingly, while food consumption rates were not found to vary between the two habitats, those individuals that were the most efficient foragers were also the earliest breeders.
So what does the work on scrub-jays tell us about the effects of supplementary feeding on the timing of breeding? If, as the work of Fleischer et al. (2003) suggests, foraging efficiency increases through access to garden feeding stations, the predictability of this significant food resource may provide the birds with a suitable cue for the appropriate timing of breeding. Of course, the supplementary food might influence the timing of breeding in other ways, perhaps by altering female body condition or behaviour. These two possibilities are known as the ‘anticipation hypothesis’, where females use food availability as a cue for the onset of breeding, and the ‘constraint hypothesis’ – where food availability constrains the female’s ability to produce eggs.
Earlier work by the same research group (Schoech & Bowman, 2001) revealed few differences in female body condition prior to breeding between suburban and wildland populations, suggesting instead that the rate of food intake or time spent foraging – both of which are shaped by food predictability – were more important. Later work on Florida Scrub-jays has attempted to look in greater detail at some of the underlying mechanisms, focussing on protein in the food taken by the birds and also monitoring the levels of key reproductive hormones. Protein is thought to be of particular importance, with evidence suggesting that wildland scrub-jay populations are protein limited – they have lower levels of plasma protein than suburban birds (Schoech & Bowman, 2003). Wildland scrub-jays feed on acorns, the tannic acids in which have been shown to reduce the ability to assimilate proteins. Schoech & Bowman provided their study populations with supplementary food of two different types; one was high in fat and high in protein (‘HFHP’), and one was high in fat but low in protein (‘HFLP’). Scrub-jays in both treatment groups bred earlier than those in an unfed control group, but birds in the HFLP group bred later than those in the HFHP group.
The researchers then looked at testosterone levels in male scrub-jays, estradiol levels in females, and corticosterone levels in both sexes. The HFHP males had higher testosterone levels