Does garden feeding shape populations?

Blue Tit, by Paul Newton
Our understanding of the impact of feeding wild birds is far from complete, but we are beginning to unravel the effects of providing foods at garden feeding stations. An important area of research has been to examine how supplementary foods shape populations through its impacts in individuals.

Feeding wild birds is a popular pastime and many of us provide seed and other foods to help our feathered friends. But what impact does all this food have? It is a huge resource and one that can increase overwinter survival and bring forward the timing of breeding, but we also know that the feeding of wild birds has been linked to the transmission of disease. Research by Kate Plummer and colleagues provides new insight into one particular aspect of food provision – how it shapes bird populations.

Kate’s research has examined the extent to which the provision of supplementary food during the winter months influences the physiological condition of individuals and populations the following breeding season. By using woodland populations of Blue Tits, Kate and fellow researchers have been able to compare the effects of providing fat, and fat plus vitamin E (an antioxidant), against a control population of unfed birds. The feeding carried out during the winter months ended at least a month before the tits began egg-laying.

Provisioning with fat and vitamin E was found to alter the composition of Blue Tit populations, such that they included birds that had been in significantly poorer condition prior to feeding. Because those individuals were found to have lower levels of carotenoids in their breast feathers than unfed birds, Kate was able to conclude that supplementing with vitamin E and fat in winter had altered the survival and recruitment prospects of these lower quality individuals; lower levels of carotenoids are indicative of poorer physiological condition. However, provisioning with fat alone was found to have a detrimental impact on breeding birds.

It appears that the provision of supplementary foods during the winter months can alter both the structure of the breeding population the following season, and the condition of individual breeding birds. Such effects may have consequences even longer term; through this work, for example, it was found that individuals with higher blood plasma concentrations of malondialdehyde (which is indicative of oxidative damage) produced offspring that were structurally smaller and which suffered from reduced fledging success.

The importance of antioxidants, like vitamin E, can also be seen from Kate Plummer’s earlier work on yolk mass.  While Plummer et al. (2013) found that winter provisioning with fat subsequently impaired an individual’s ability to acquire, assimilate and/or mobilise key resources for yolk formation, this was not the case where vitamin E was also included in the food presented. A high fat diet, such as that potentially obtained from the food provided at garden feeding stations, may well increase the requirement for antioxidants in order to combat the greater levels of oxidative damage associated with a diet rich in fats. Clearly, there is still much to learn about how the provision of supplementary food affects wild birds and their populations.

Related publications:

Plummer, K.E., Bearhop, S., Leech, D.I., Chamberlain, D.E., Blount, J.D. 2018
Effects of winter food provisioning on the phenotypes of breeding blue tits. Ecology and Evolution.
Link to publication (DOI: 10.1002/ece3.4048).

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