Starar og hestar

Abstract

The habit of the common starling (Sturnus vulgaris) to associate with grazing horses is spreading in Iceland. When asked I found that many thought the birds were picking parasites from the skin of the horses. I found no references to scientific papers for this association of the two species on the internet. However, the behaviour of starlings to forage for invertebrates close to cattle when grazing has been studied. In those papers the benefits for the cattle was hardly mentioned. Photos of starlings sitting on the back of horses are not many on the internet, interestingly though the majority from Iceland. Some photos of starlings perching on red deer, elk, bison, reindeer and cows do exist. To get some idea of the functions of this behaviour, preliminary behavioural observations were carried out in Eilífsdalur, SW-Iceland, on 9 days in August and September 2018 and 2020 in a spacious pasture with a group of 24 horses. The method Ad Libitum was used to be able to describe the general behaviour of the starlings at the site and the reactions of the horses to the birds. Instantaneous sampling was used to estimate time budgets of the birds (22 scans) and to collect data to test (14 scans) if the colour of the horse was irrelevant for the birds when they perched on their back. The birds were very movable and agile: they frequently formed large flocks that split up and joined up, flew up in the air or landed close to the horses. Sometimes they settled on the horseʼs backs (up to 25/horse). Looking closely at their behaviour it became clear that they were not feeding on parasites (lice) or skin flakes (one clear exception). Rather, they seemed to use the horses as perches to spot where other birds were feeding or for a place to rest, preen the feathers or for interactions with other birds as well as to be on the outlook for predators. They spent 73% of their time foraging for food (berries and invertebrates around the horses and in the horse droppings). They also stood still (13%), preened their feathers (7%), argued over horse droppings (3%) and spent 4% of their time perching on a horse. Most commonly only one bird perched on a horse at the same time. Interestingly the colour of the horse was relevant – the birds preferred the palest horses. The horses did not shake their body or tail to get rid of the birds nor were they aggressive towards them. This supports the idea that this association might be a case of commensalism – the birds benefit by getting access to more food while the presence of the birds for the horses is of no importance. However, in the evolutionary past when the horsesʼ habitats were large plains and predators were a constant threat to the foals, an association with birds which spot predators quickly19 might have been advantageous for the horse as well as acting as deterrents for biting insects–hence a case of mutualism. Although such a predator threat is of much less significance at the present time, the genetic basis of a behaviour of such a protective nature is very unlikely to disappear from horse populations.