Great Grey Shrike - Behavior

Behavior

This species is territorial, but likes to breed in dispersed groups of a good half-dozen adults. It is not known to what extent the birds in such groups are related. In the temperate parts of its range, groups are perhaps 5 km (3.1 mi) apart, while individual territories within each group may be as small as 20 ha (49 acres) but more typically are about twice that size. In less hospitable climes, territories may be more than 350 ha (1.4 sq mi) large. Throughout the breeding season, in prime habitat territories are held by mated pairs and single males looking for a partner. In less productive habitat, "floaters" hold territories more ephemerally. This leads to shifts in population density between regions, as "floaters" move between groups of territorial birds in search of a bountiful unclaimed territory to settle down and/or a partner to mate with. On the wintering grounds, pairs separate to account for the lower amount of food available at that time, but if both members migrate they tend to have their wintering grounds not far apart. As it seems, once an individual Great Grey Shrike has found a wintering territory it likes, it will return there subsequently and perhaps even try to defend it against competitors just like a summer territory. Throughout the year, the birds regularly but briefly move through a range up to three times larger than their territory; this is tolerated by territory owners in winter more easily than in summer, and the parts of Europe where all-year residents and winter visitors co-occur typically have population densities around 8 birds/km2 (about 30 per square mile) and occasionally more in winter.

Before and after the nesting season, groups of breeding birds will sometimes initiate gatherings; these seem to occur at the boundary of the group's combined range or in the unclaimed land separating it from neighboring groups. The initiation signal is a conspicuous display flight given by a bird surveying its territory: it spirals tens of meters/yards high into the air, usually briefly does a fluttering hover at the top of the spiral, and then glides down. Group neighbors will respond by performing the same type of flight, and eventually about half the group's members will depart to the meeting location where they will spend several tens of minutes – sometimes more than one hour – chattering, calling, duetting, and excitedly moving about the meeting site (which typically is some small tree or shrubbery). In winter, birds will often assemble in small groups and roost together, particularly to keep warm during the night; this is apparently not initiated with a specific assembly display however.

The flight of the Great Grey Shrike is undulating and rather heavy, but its dash is straight and determined. It is, as noted above, also capable of hover flights, which last briefly but may be repeated time after time because of the birds' considerable stamina. It will usually stay low above the ground in flight, approaching perches from below and landing in an upward swoop. In social interactions, birds signal an aggressive stance by a bold upright posture, fanning and then flicking the tail and eventually the wings also as the bird gets more excited. It signals its readiness to strike at an intruder by shifting to a horizontal pose and fluffing its feathers, raising them into a small crest along the top of the head. Birds appease conspecifics by head-turns away from them (if close by), or by imitating the crouching fluttering pose and calls given by fledglings begging for food (if sitting father apart). The submission gesture to prevent an imminent attack by a conspecific is pointing the beak straight up.

North American fledglings apparently have one moult in their first spring only, in which they acquire adult plumage. Eurasian ones moult part of their juvenile plumage before their first winter, and the rest in spring. Adults moult on their breeding grounds before going on migration, or before the depth of winter if they are resident. Sometimes adults also seem to moult some feathers before attempting to breed. As moult requires a considerable investment of energy, some significant evolutionary benefits to offset this are likely. Reducing feather wear and parasite load, moulting can make a bird more physically attractive and healthy, and may thus increase its chance of successful reproduction. The phenomenon is not well understood, however.