Tree Sparrows - Lifecycle
Tree Sparrows are sedentary birds and once they are settled in a colony will usually remain faithful to it for life. Outside the breeding season the adults and new young will forage further afield but nevertheless tend to remain in a home range of a few square kilometres.
In the autumn young birds return to the colony with the adults, filling the vacancies left by mortalities and eventually pairing up to breed themselves. Some young birds, unable to find a place in the colony, may migrate (occasionally along with adult birds) further afield but this only involves small numbers.
Breeding
In the breeding season, Tree Sparrows form pairs for life and these pairs remain faithful to their nest site. That said, with the low survival rate, most pairs are formed are formed by mate replacement as one of the pair disappears.
The nest site – either a naturally occurring hole (eg in a tree) or a nestbox – is central to the pair’s formation, and all breeding activities (nest building, incubation, brooding and feeding of the young) are shared by both birds. Most pairs will raise two broods in a season but some will go on to raise a third. Typically 60% of the eggs laid go on to fledged chicks and there is a correlation between latitude and breeding success and productivity.
Movements
Tree Sparrows are largely sedentary and spend most of their lives within a few kilometres of their breeding site. Birds that disperse from their birthplace mostly settle to breed within about 10km. There is some evidence that a very small number of birds (<5%) from these sedentary populations migrate south. Occasional large-scale eruptive movements have also been recorded from time to time.
Numbers
Tree Sparrow colonies do not grow beyond a certain size with colonies consisting of 10-50 pairs and, in years of high population growth, excess birds move on to join other colonies or set up new ones. Outside the breeding season Tree Sparrows form larger foraging flocks with birds from other colonies and range outside the ‘home area’ in search of food.
Mortality rate is high in Tree Sparrow populations with just 15-20% of the young surviving into their first breeding season and only 1-2% surviving into a third breeding season. Ringing recoveries have shown that a few birds survive until they are 5 years old.
In times of high population, new colonies are established as the excess young move into areas where there are suitable nest sites. In a fully functioning colony, the breeding population is made up of approximately 35% of last year’s adults, 60% of incoming Tree Sparrows from nearby colonies and 5% of juveniles born in the colony.
Feeding & Daily Routine
The Tree Sparrow is a sociable species, breeding in loose colonies and associating in small flocks the remainder of the year. Tree Sparrow days are spent food seeking (mostly on the ground), bathing (dust & water) and roosting in groups in thick trees, bushes and reedbeds. In more severe winter weather Tree Sparrows may return to roost in nest holes.
The principal diet of adult Tree Sparrows is seed – typically from common weeds of waste places and cultivation, such as wild grasses. Seed is also taken from cultivated cereals but to a lesser extent. In contrast the young are fed almost exclusively on a diet of insects with beetles and moth larvae making up the bulk of the diet.
Source for Tree Sparrow information:
J. Denis Summers-Smith, The Tree Sparrow, (Cleveland, 1995)
A pair of Tree Sparrows preparing their nest - photo by Simon West
A young Tree Sparrow - photo by Rupert Evershed
An adult Tree Sparrow at a feeder - photo by Rupert Evershed