Friday, 18 December 2009

The Sparrowhawk

Hello all,

Firstly I would like to say sorry for my lack of updates. My degree at University is going well, and I'm enjoying it, especially the perks, of meeting some really interesting people, gaining knowledge, and borrowing the camera equipment :)


I've just broke up for my 4 week Christmas holiday, so I will expecting to be down at Holywell most days, for now I will leave you with my first 2000 word Ecology Assignment, which is based on the Sparrowhawk Accipiter nisus. Hope you enjoy it, and most likely see you whilst I'm knocking about.


Cheers,
Cain


Ecology
Assignment 1
Essay on the ecology of a chosen species

The Sparrowhawk Accipiter nisus


Juvenile Female Sparrowhawk, Accipiter nisus, on Quarry
Copyright, Cain Scrimgeour


In this essay, the ecology of the Sparrowhawk Accipiter nisus will be looked at, its distribution, limiting factors, habitat, reproductive strategy, behaviour, the ecosystem it exists in, population ecology, and its current and past threats will be looked at in some detail.

The Sparrowhawk is a small-medium sized bird, it has a great size difference between male and female birds; male birds vary between 27-30cm (from head to tail), weighing at about 150 grams, whereas the females between 35-37cm weighing about 300 grams. This difference is common in most birds of prey and a few other bird species although exaggerated in Sparrowhawks to a great degree. Due to this large size difference each sex has a different ecology in terms of hunting habitats and prey species. The structure of both sexes is similar; the species has a slim body, on long thin legs with long slender toes, short broad wings and a long tail, it is built to negotiate thick woodlands, and catch bird species, mostly songbirds. Their build allows for fast pace flights as well as an amazing degree of manoeuvrability, allowing them to fly through thick areas of woodlands, suburban gardens and other difficult flying habitats. In identifying the species, both sexes are similar although different, apart from the obvious size difference the female can be recognised by a brown/grey upper surface, with a whitish, dark barred under side, in males the upper surface is more of a blue/grey, with a slightly more orange tinge under side, also with dark horizontal barring. The juvenile plumage lasts for the first year and lacks the grey of the adults with a brown upper, and also a buff under side with barring although more like diamonds/hearts marked across the chest to form what appears to be barring. The plumage of all ages can vary significantly from one individual to the next.

The distribution of the Sparrowhawk is vast, as long as there are suitable woodlands the majority of locations are suitable (in the Palearctic region), it can be found throughout Europe, Asia, and into North Africa. In this range certain populations are migratory, mostly in the northern populations, where temperatures drop to extreme lows in the winter, and a food supply cannot survive. The species has not been recorded south of the equator. Throughout this region there are a number of subspecies although similar, ‘What slight variation exists is mostly clinal (gradual) though six subspecies are recognised, including four which are geographically separate in the breeding season’ Newton (1986, p.41) although Sparrowhawks Accipiter nisus, can be found throughout Britain, from cities to the countryside, they are not present in high areas, where nesting habitat is limited.

The only true limiting factors of Sparrowhawk Accipiter nisus distribution is the lack of suitable breeding habitat in males; and a good mate for females, the process of nesting habitat is highly selective. Sparrowhawks breed, hunt and roost in woodland (although hunt across a wide range of habitats), preferences for sites have been observed, woodlands which are open so flights paths are clear, but covered to provide security. Therefore there is a period in a wood/forests growth where it is attractive and suitable for Sparrowhawks to nest. They usually prefer large woods, with young tree’s ‘2 to 4 metres apart’ Newton (1987, p.4). Although poorer suitability woods will be used if there is a lack of suitable nesting habitat, through my own observations on the green belt land surrounding Newcastle, it is clear that Sparrowhawks will make use of what woods they can find to provide a nesting habitat. Females are at a greater number to males in any given population, therefore finding a good, reliable mate with an established territory is difficult.

As stated above the Sparrowhawk main habitat is woodland, which is vital for breeding and roosting, although a wide variety of habitats are used for hunting purposes. The Sparrowhawks wing, tail and body structure makes it perfectly adapted to fly through woodland, weaving in between trunks and branches, and allows it to chase down fast flying songbirds within this habitat, but this manoeuvrability also allows to hunt in a variety of other habitats, including town and city gardens, were the agile flight allows the use of fences and houses to gain access to its prey. Sparrowhawks have been observed hunting in many habitats, from reed beds and sand dunes, to farmland and moors where a variety of prey species, almost entirely birds can be hunted.

Sparrowhawks breed when there is a peak in their prey, which occurs due to their breeding cycles, they breed between mid spring and late summer, due to the increase of food which they need to survive e.g. plants and insects, due to the increased amount of light available in the day. This therefore means Sparrowhawks breed between April and August. Sparrowhawks begin building nests between the months of February and April, for the males to keep a female within a territory he has to provide a food supply for her, this will keep her near to the nest site for greater lengths of time, therefore holding the nest site from other females in the area. For the male to provide a substantial food supply his home range and hunting experience must be at a high level, the less time spent hunting the more time which could be donated to the female, continually feeding her at the nest site. At the beginning of the breeding season all the females of an area may overlap a single males territory amongst several others, allowing choice of the best provider, and nesting site within this range. ‘We could thus infer that any inability of the cock to provide food resulted in the hen wandering’ Newton (1986, p. 152). When a female made the choice to stay within a certain male’s territory and nesting site to breed, it then needs to defend the territory, food source, male and nesting site against any other wandering females, which would provide significant competition. Any females coming into contact with the resident female or nesting place are chased out of the area by the female, if the threat comes from above a series of territorial flights will begin. This is normally enough to deter any competition.

Ian Newton has distinguished six phases in behaviour between the pair before eggs are developed. ‘ (1) the attraction of birds to nesting places and potential mates to one another; (2) mutual roosting on the nesting area, mutual calling in the early morning, and aerial displays; (3) the feeding of the hen by the cock; (4) nest site inspections and stick carrying; (5) nest building proper; and (6) copulation’ Newton (1986, p.156). This serious of phases works in a succession with each step being completely necessary before the step after is followed, each individual pair can take varying amounts of time to complete the steps. The food supplied by the male to the female is a vital step, if there is a lack of food then nest building cannot begin or progress, therefore copulation and breeding will not occur. Courtship feeding can be seen in a wide variety of birds, in the Sparrowhawks case the male brings food items, one at a time to the female, either to her perch, or in an aerial pass called a foot pass, which is common amongst birds of prey, the Peregrine Falcon Falco peregrinus being a prime example. This courtship feeding allows the female to put on sufficient mass for the purpose of egg development, at a point before the eggs are layed the female may become dependant on the male supplying food, this is the beginnings of where a good, experienced mate allows for reproduction, and the fledging of offspring. If the male fails to provide the female may not lay, and breeding attempts may fail for that year.

Nest building begins between January-April, more commonly late March or Early April, as eggs are often laid in May. Both Male and Female birds construct the nest, although main construction and structure is the males job, with the female adding finishing touches, such as the lining, smaller twigs and flakes of bark, but also leaves and needled branches. It is located mid canopy in the woodland, not quite in the canopy but not in the ground, this makes it difficult to be seen, blending in to the leaves above when looking from below, and blending into the ground foliage we looked at from above. The nest may be built on top of old nests from other species such as Woodpigeons Columba palumbus and Carrion Crow Corvus corone corone, or from past Sparrowhawk nests, which haven’t been destroyed by winds. This structure provides a safe place for eggs and young to be incubated and raised. The eggs, once laid, are a brown/red colour with a mottled pattern, which are extremely variable from one pair to the next, ‘ generally consisting of spots and blotches, they may be clustered together at one end in a group or in a solid cap, they may form a zone or they may be scattered more generally over the shell. Quite often one egg in a clutch will be almost unmarked.’ Evans (1972, p. 144). The female will lay between 4-6 eggs, on alternate days with incubation tending to start on the second last egg, allowing for a spread hatch. The female will incubate the eggs for 32-34 days, in this time the male will provide her food, which is eaten on a close to branch, therefore if the male cannot provide, she may have to hunt for herself increasing the chance of predation of the eggs. The male continues to provide once the chicks have hatched, until they are old enough to be left, the prey item will be taken to a plucking post, where the male will remove the feathers, before passing to the female, which will one by one tear small pieces of meat off and feed each individual white downy chick, which have fully developed eyes, and are able to take food from there mother. At this time and throughout the rearing stage the mother will continue to look after the chicks, ‘The young are brooded almost continuously for the first seven or eight days, then progressively less until twelve to fifteen days, thereafter chiefly during rain.’ Newton (1987, p.18), and nest, mostly protecting the chicks from the weather predators, and removing uneaten food/remains. The nest remains clean in terms of faeces due to the innate instinct of the chicks to back up to the edge of the nest, before releasing their waste, this response keeps the nest clean and bacteria free, the mother may still add further lining material to the nest.

Once the female can also provide, the young receive about 8 meals per day, allowing them to grow rapidly. The females gain weight at a greater rate than males, possibly due to their increased size, but the males develop flight feathers earlier, with flight being achieved at 26 days, and females 30 days old. Once young have left the nest completely they stay in the area, and are further fed by the parents for 3-4 weeks, once this period, which will involve flight development has came to an end the young will move out of the area and will have to hunt to prey for themselves.

The Sparrowhawk Accipiter nisus, sits near to the top of the ecosystem it exists within, it is a first-degree carnivore preying almost entirely on small-medium sized birds, although small mammals are included within its diet. As an adult, the larger Goshawk Accipiter gentiles, is the only natural predator within Britain, although as eggs, chicks, and fledglings opportunistic predators include, Tawny Owl Strix aluco, Jay Garralus glandarius, Magpie Pica pica, and Pine Marten Martes martes. Humans also persecute the Sparrowhawk, through shooting and the use of pesticides. The use of organochlorine pesticides had a severe effect on Sparrowhawk populations, this effected the Sparrowhawks through there prey, which fed upon contaminated plants, as it is absorb by fat it is passed on from one trophic level to another, with concentrations increasing at each, so carnivorous animals acquire a greater amount of the toxin than that of the primary consumers. Although this pesticide can stay within a ecosystem for a great length of time, causing the thinning of egg shells, it is thought that the Sparrowhawk population has completely recovered.

Ian Newton established an idea of hunting behaviour through observations and radio tracking; the techniques were used in the relevant environment that allowed greatest productivity. ‘Short-stay-perch-hunting’ Newton (1986, p.102) involves the bird flying through a wooded area from one tree to another, scanning the area at each step, this was found to be the commonest method for searching for prey. ‘High soaring’ Newton (1986, p.102) and stooping, this involves a bird soaring to a great height out of site, scanning the area below for suitable prey species, and stooping, closing its wings and dropping down to grasp its prey, this behaviour is usually used on flocking birds, I have witnessed this behaviour many times near my home, mostly at Starlings Sturnus vulagris and Woodpigeon Columba palumbus. ‘Contour-hugging flight’ Newton (1986, p. 102), this behaviour involves the bird making use of the terrain it hunts, mostly seen in housing estates where the bird will make use of hedgerows, fence lines and buildings to catch prey species off guard, the bird may continuously switch sides, these routes can become regular if productive. I also witnessed this hunting behaviour at my home, but also as a falconer I have had the opportunity to fly a female Sparrowhawk at Corvidae, once the bird had seen the quarry and was released it could easily be seen that contour-hugging flights were being established if there was a suitable environment ahead, such as a line of tree’s or a hedge line, this type of flight was very effective whilst flying a captive bird. Other hunting behaviours include, ‘still hunting, low quartering, hunting by sound, and hunting on foot’ Newton (1986, p. 102-103).

Due to the increasing human tolerance of Sparrowhawks Accipiter nisus, its distribution within the UK will increase, due to the large amounts of tree’s which are planted within urban areas, providing more vital nesting areas. Through the decline of farmland birds at present, I believe Sparrowhawks will become more common within urban areas, following their food source, and less in farmland areas, which could effect the overall population, unless conservation efforts are increased to help save farmland birds.


References

Newton, I. (1986) The Sparrowhawk. The Bath Press, Avon.

Evans, G. (1972) The Observer’s book of Birds’ Eggs. Frederick Warne & Co Ltd, London.

Newton, I. (1987) The Sparrowhawk. Shire Publications Ltd, Buckinghamshire



3 comments:

  1. Hi Cain. Welcome back.:-) Good to see that you have been busy! :-) I found nesting Sparrowhawks on patch this year and now wish I'd taken a little more time to keep an eye on them. As you mention, the nest is not easily found even in a small patch of trees as this one was.
    I guess you will have seen that Ian Newton is giving the annual lecture for the NHS on 8th January and it is about his study of Sparrowhawks. I'm wondering if you will be around to attend?
    Now remember to get our day out in the diary.:-) Cheers Brian.

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  2. Interesting stuff Cain. We have a good one kicking around Holywell at the moment. Showing well everyday at the hide. Hope to catch you soon.

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