Juggling a Junk Food Diet: Urban Birds Go on a Diet on Weekends Due to Less Availability of Junk Food

Cities, a smorgasbord of snacks

Cities are full of food! Many scientists hypothesize that urban animals thrive due the consumption of human food. For example, rats were shown to have been exploiting human food as early as 1790. Human sources of food often are more abundant and predictable than natural food items. Additionally, animals spend less time searching for food and have access to food in times of natural scarcity (like winters or droughts). Some animals that use human food sources grow in population size, decrease their territory, increase their survival and/or have greater number of babies. As urbanization continues to develop even the most iconic birds, Darwin’s finches, are developing a taste for human food.

It’s hard to snack on the weekends

Figure 2: Starling at the university looking for food near a trash bin. Photo by Miqkayla Stofberg

People eating provide an important source of food. Many animals eat the crumbs of food people spill or throw in trash bins. However, human traffic can vary during the week. Because of this the abundance of human food can vary on a daily basis. A new study  by Miqkayla Stofberg et al. set out to evaluate how human activity influenced the diet of the red-winged starling (Onychognathus morio) in the University of Cape Town in South Africa (Figure 2).

In their study, the researchers compared aspects of birds diet and foraging behaviors on days of high human presence (HHP) and low human presence (LHP). They found that human food diet of starlings consisted mostly of apples, followed by bread, muffins, and crisps.  Starlings ate more human food on days with more humans (HHP). However, the amount of total food eaten did not change (Figure 1). Additionally, birds gained more body mass on days when they ate more human food.

Figure 2: Panel A shows the human food intake of birds on days with high human presence (HHP) and days with less humans (LHP). Panel B shows the amount of food birds ate measured in “beakfuls”.

The “see food” diet, birds eat what they “see”

Other studies have documented diet and foraging behavior changes between sites that vary in human activity. This study is interesting because they show diet changes within the same site. Rather than spending more time looking for food, birds eat more natural food when human food is scarce but capitalize on human food resources when available. This finding highlights the importance of natural vegetation within cities as important food sources. This is important because, as this study shows, the abundance of human food may vary with human traffic. Additionally, human food may not be available to fulfill all of the animals dietary needs. For example, human food may lack important antioxidants. It’s important that we continue to monitor the long term implication of human food snacking. For many species, consumption of human food can bring negative consequences such as the spread of disease and parasites due to crowding of more individuals and changes to the gut content microbiomes. And, as we’ve started to see in Darwin’s Finches, may even lead to morphological shifts and niche degradation. If you see any animals eating human food be sure to send us the picture. They may even make into the urban observation of the week!

For more information on this topic read the paper and visit the project’s Facebook page where they have some fantastic photos and videos Header photo taken by Kervin Prayag.

Stofberg M, Cunningham SJ, Sumasgutner & P, Amar & A (2019) Juggling a “junk-food” diet: responses of an urban bird to fluctuating anthropogenic-food availability. Urban Ecosyst. doi: 10.1007/s11252-019-00885-3

To see some of the other studies on this topic check out these papers:

Chard M, French K, Martin J, Major RE (2018) Rain drives foraging decisions of an urban exploiter. PLOS ONE 13(4): e0194484. https://doi.org/10.1371/journal.pone.0194484

Jeremy Lyons, Gabriela Mastromonaco, Darryl B Edwards, Albrecht I Schulte-Hostedde, Fat and happy in the city: Eastern chipmunks in urban environments, Behavioral Ecology, Volume 28, Issue 6, November-December 2017, Pages 1464–1471, https://doi.org/10.1093/beheco/arx109

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