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Three of the more pressing issues in bird-feeding research are the link between bird-feeding activities and transmission of diseases among birds (e.g. Bradley and Altizer, 2006), the degree to which anthropogenic food serves as a dietary supplement to a diverse array of food items consumed by birds and the degree to which feeders create dependency among bird populations (Brittingham and Temple, 1992; Jones and Reynolds, 2008). Each of these issues, disease transmission in particular, has been considered in existing studies of the impacts of anthropogenic food on wildlife. A recent meta-analysis by Becker et al. (2015) includes excellent evidence-based discussion of the importance of fully evaluating the costs and potential negative impacts of human alteration of the foraging ecology of wildlife and its link with increased disease transmission. Given that some studies show positive impacts of bird-feeding activities, whereas other studies show costs or negative impacts, combined with the knowledge that most bird populations are in decline, it is absolutely crucial that all investigations of the impacts of anthropogenic food on bird health consider benefits and costs alike.
Several fundamental questions about wild bird feeding remain. In particular, few studies have examined the impact of supplemental food on wild bird populations, including how bird feeding influences the health and energy demands of individual birds and may change the overall bird community (although see Brittingham and Temple, 1988; Geis and Pomeroy, 1993; Pravosudov et al., 2001; Schoech et al., 2004). From spring 2011 to spring 2014, we examined how bird feeding impacts wild birds by evaluating the health of individual birds with a broad range of metrics, including body condition, stress, antioxidant levels, nutritional condition, immune function and disease, by comparing forested sites with and without feeders.
Defining avian health and choosing relevant metrics can be challenging. In many cases, physiological responses are context dependent, and in general, it is unlikely that any single measure is truly representative of the health of a free-living bird. For this reason, we used multiple metrics that measure a diverse array of physiological functions. Body condition has been defined in a very broad sense to indicate the physical make-up of a bird that confers the ability of an individual to cope with present and future physiological stress, and therefore, the ability to enhance fitness (Carrascal et al., 1998). Mass alone is unlikely to serve as a reliable indicator of condition, as an animal can be heavy because it is structurally large or because it is carrying abundant fat or protein (Dobson, 1992); therefore, using measures that consider the structural size and mass, as well as storage of macronutrients such as fat, are important. In general, wild songbirds have very little fat because they need to remain light. However, fat reserves can be important in buffering an animal against fluctuations in food supply or serving as fuel for energetically demanding flight, as in migrating birds (Blem, 1990).
Each bird completes at least one moult per year, dropping each of its feathers and growing a new one in its place. During the regrowth process, a visible growth bar is developed with each day of growth until the feather is fully developed. The assessment of feather growth bar length, or ptilochronology, has been validated in captive and wild birds as a reliable indicator of nutritional condition (Grubb, 1989). Therefore, is likely to serve as a good measure of the impacts of anthropogenic food on bird nutritional condition.
The singing of this song threw the animals into the wildest excitement. Almost before Major had reached the end, they had begun singing it for themselves. Even the stupidest of them had already picked up the tune and a few of the words, and as for the clever ones, such as the pigs and dogs, they