Temporal avoidance as a means of reducing competition between sympatric species

Competition between species has been put forward as a possible reason for the declines seen in some bird species, including Marsh Tit and Willow Tit, but confirming a causal link between competition and the changes in a population can be challenging. This is particularly true where that competition comes indirectly, from one species depleting resources important to another. It can be possible, however, to infer the existence of negative interactions between species through observations of their behaviour. In this study, the authors examined the interactions between three tit species, encountered in mixed-species groups at feeding stations in Wytham Woods, Oxfordshire.

Tits are known to aggregate in mixed-species groups outside of the breeding season, forming foraging flocks that can be beneficial to group members in finding food or reducing predation risk. However, being part of a mixed-species group ceases to be beneficial when the costs of doing so outweigh the benefits. Such costs are rarely distributed evenly, and those species and individuals with a lower social or competitive status typically carry the greatest burden.

The Wytham Woods tit populations have been well-studied and individual birds each carry a tag, called a passive integrated transponder, which has a unique identification code. The code can be read by a special antenna, located on the entrance of each of the feeding stations provided throughout the wood. Each visit to a feeding station is logged, and from this it is possible to build up a picture of which individuals were together at a feeding station at any given time.

The researchers were able to secure information for nearly 150,000 flocking events and determine some 420,000 records of an individual Marsh Tit’s presence or absence within these flocks. The results revealed that Marsh Tits were less likely to join larger mixed-species flocks than smaller ones, and they also accessed food less frequently when in a larger group than they did in a smaller one. Additionally, Marsh Tit numbers within groups were found to decline through the day, and over the winter period, while the numbers of Blue Tits and Great Tits increased.

The study suggests that Marsh Tits avoid foraging alongside the socially and numerically dominant Blue Tits and Great Tits, but that they are limited in the extent to which they can do this. This means that the behavioural responses of foraging Marsh Tits – avoiding larger mixed-species groups – confers only a partial reduction in the levels of competition experienced. 

One way that Marsh Tits might be able to reduce competition is through their habit of caching food items, and this may help to explain why the numbers of Marsh Tits in foraging parties declined through the day. If individual Marsh Tits can cache some of the food collected early in the day, they can then exploit these caches later in the day, avoiding additional competition; Blue Tits and Great Tits do not routinely cache food.

This is the first study to provide empirical evidence of a significant negative effect of dominant generalist species (Blue Tit and Great Tit) on the foraging behaviour of a subordinate specialist (Marsh Tit), something that has implications when considering the ways in which the availability of natural resources is altered by human activities.

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