When bluebirds and swallows do battle over nesting sites, prior ownership counts. Bluebirds are much better at defending a sweet nest that is already in their possession than they are at claiming one that is open. A study on this phenomenon could shed light on many other animal conflicts, and even one of the great sporting puzzles: why do teams play better at home?
Tree Swallows (Tachycineta bicolor) and Mountain Bluebirds (Sialia currucoides) nest in similar tree hollows. Finding the right hollow is essential for both birds if they are to successfully raise young, and the species often fight intensively for possession.
Dr Karen Wiebe of the University of Saskatchewan set up nesting boxes next to each other in a grassland site, otherwise devoid of nesting places, at Riske Creek, British Columbia. Mountain Bluebirds will not build a nest within 100 meters (330 feet) of other bluebirds, so while one box was claimed by a Mountain Bluebird pair, the other was always inhabited by swallows.
Wiebe then either blocked up the bluebirds' box, blocked the swallows' box, or removed both and put in a new box. In each case, the two sets of birds did battle over the precious remaining resource.
When the two birds competed on equal terms the swallows won 70 percent of the time, despite being smaller and lighter than their rivals. When swallow pairs had prior ownership of the box they got to keep it a statistically similar 66 percent of the time. Yet for bluebirds, while possession isn't “nine-tenths of the law” it is almost eight-tenths – they won 77 percent of the time when on their home turf. In Auk and Condor Wiebe notes that previous resource owners defeating usurpers is a “prevalent pattern in animal contests.”
The bluebirds' defensive success may be down to the fact this is a far more familiar situation. Mountain Bluebirds arrive at the nesting grounds earlier in spring than Tree Swallows and get first pick of nesting sites. As with so many other things, global warming could change this, disrupting the delicate balance between the two birds.
Wiebe wrote: “Why the success of Mountain Bluebird increased significantly when they had prior ownership is not entirely clear.” But she hopes further investigation could help shed light on this phenomenon more broadly. In many other contests, prior possession may indicate some advantage such as strength, knowledge of the terrain or placing a higher value on the resource. However, if bluebirds only hold their nests through early arrival, something else must be going on, and this might be applicable to other species.
The outcome may even be of interest to sports fans, helping explain why across so many types of contests there is a home ground advantage.