Do coal tits lose body mass if chased by predators?

Issue 18, July 2000

Coal tits (Parus ater) are small birds (about 10 - 11 cm in length), see Figure 1, that are found in woodland, especially coniferous woodland. They are insect feeders and use their bill to search for food among the cones and needles. They need to eat not only for energy and growth but also to store fat as a reserve to use in order to withstand cold nights and low winter temperatures and for rapid take off if danger threatens. It has been suggested that as the risk of predation increases it would pay a bird to reduce its body mass since a lighter bird has greater manoeuvrability and a faster take off. So, do birds reduce their body mass if the predation risk is high?

Figure 1. A coal tit.

A recent study* in Spain set out to test this using coal tits. The birds were caught in the wild, given leg bands to allow identification and then released into a large outdoor aviary in a clearing in a forest. Pine and cedar branches were placed inside the aviary which the birds used for perching, resting and as shelter. The tits had ad libitum access to food and water in the aviary.

To investigate the risk of predation in this study the researchers entered the aviary and tried to catch the birds by hand - so the predators were humans. At the start and end of each day the birds were weighed so that any change in mass could be detected. Twelve birds were used in the study and they were randomly assigned to either the control group (no attempts being made to catch these birds) or the experimental group (in which 4 attempts of 6 minutes per day were made to catch the birds. The birds were all released back into the wild at the end of the investigation.

* Carrascal, L. M. and Polo, V. 1999. Coal tits, Parus ater, lose weight in response to chases by predators. Animal Behaviour, 58, 281 - 285.

1. The researchers were interested to see if, as predicted by models, the body mass of birds changes with the risk of predation, though in this case the predators were human. To do this they caught wild birds and studied them in an aviary. Why do you think they used wild birds rather than ones reared in the lab? (1)
2. The birds in the study were kept in an outside aviary in a small forest clearing. What advantage would an aviary in a natural setting have over an aviary in a typical laboratory setting? (1)
3. When the experimental sessions took place, the 12 birds were divided into two groups: an experimental group and a control group. Explain the nature of these two groups. (2)
4. During the Predation risk experiments, the experimental group of birds were chased around the aviary until the experimenters caught them. Why did they not use a real predator, a hawk for example, in the investigation? (2)
5. At the start and end of each day each bird (in both the experimental and in the control groups) was captured and weighed on a balance. What type of data was collected? (1)
6. After being weighed the birds were placed in a box overnight and then re-weighed in the morning before being put back into their aviary. The data from these weighings are illustrated in Figure 2. Explain the trend evident in the graph. (2)

Figure 2. Mean change in the body mass (g) of coal tits during the day. Twelve coal tits were used: 6 in the experimental condition first, then in the control condition (open circle): 6 in the control condition first, then in the experimental condition (closed circle).


7. What is a confounding variable? (1)
8. Suggest one confounding variable that may have been influential in this particular study. (1)
9. In some studies stuffed birds of prey have been used as predators to see what effect this has on the behaviour of the prey species, such as coal tits. Outline one advantage and one disadvantage of using a stuffed predator in such studies. (2)

[We are grateful to Academic Press for permission to use Figure 1 page 284 from the article by Carrascal, L. M. and Polo, V. in Animal Behaviour, 1999, 58, 281 - 285 and Figure 4 (e) page 207 from the article by Ishiwaka, R. and Mori, T. in Animal Behaviour, 1999, 58, 203 - 209.]