SIMON ANDERSSON                                                                                               2007-04-16

WILDLIFE MANAGEMENT ELECTIVE COURSE

 

 

The goshawk, Accipiter gentilis, is a magnificent bird of prey found in most parts of the northern hemisphere, from Mexico in the south to Canada in the north, where it mainly lives in forest habitat, feeding on smaller mammals and birds. It may catch prey up to the size of a hare or a capercailzie. Older individuals have a characteristic cross-banded black and white coloration on the chest and abdomen. Back and dorsal aspect of wings has a pigeon-grey colour, the wide stout tail has similar pattern to the abdomen. Younger birds may instead have a brown-dappled back, and may be confused with a female sparrow hawk. The female is bigger than the male with height at some 60cm and a wingspan at around 120cm, the male on the contrary only reach a height at 50cm and a wingspan at 100cm. The goshawk pair builds a nest from brushwood and bedded with a mix of mosses, lichens and grass. They have different nests in their territory to which they are faithful. Each nest may be used up to three years in a row and primarily the male adds structures to the nest every year, more or less analogous to Homo sapiens.

 

In a paper published in Journal of Animal Ecology by Oliver Krüger the age at first breeding and fitness in the female goshawk is discussed. The study, which was conducted on data, collected from a 250km² investigation area in Germany between 1975 and 2004 on some 74 female individuals, reaches the conclusion that the optimal age of first breeding is three years in goshawk. Which is very interesting because this age is much later than what is physiologically possible. This is opposite to e.g. the roe deer which start to breed as soon as they mature sexually. A similar delayed breeding trait is found with us humans, as we do not see many thirteen-year-old mothers, although it is possible. Not a topic of this assignment but interesting.

 

Before this phenomenon is discussed in this assignment, mechanisms explaining such a trait should be considered. There are four hypotheses, presented over the years, trying to explain this behaviour. The differential mortality hypothesis (Nol & Smith 1987) suggests that poor-quality individuals die at an earlier age, leaving good-quality individuals to breed at a later age. The delayed breeding hypothesis (Hamann & Cooke 1987) proposes that high-quality individuals delay first breeding and recruit into the population at a later age. The constraint hypothesis suggests that individuals improve in reproductive performance through accumulation of breeding experience (Woller et al. 1990) or more efficient foraging (MacLean 1986; Catry & Furness 1999). The restraint hypothesis (Williams 1966) predicts that reproductive effort increases with age because residual reproductive value increases.

 

As mentioned earlier the data was collected in Germany and has been used in many projects concerning the goshawk and can therefore be considered reliable. Goshawks moult their feathers every year and a trained eye can by study of these feathers give a very accurate classification of age and individual. The brood size and breeding success was determined by ground-studies of the nests, and each nest was visited at least three times per year. To measure fitness two methods were employed. Lifetime reproduction success (LRS) is the total sum of offspring in a lifetime of an individual, and l_ind  is a measure of how a given phenotype propagates in a population. The last thing to estimate was whether a territory could be considered good or bad. A territory was classified as good if it was used often, and a bad one was used less frequent.

 

When the data was studied it could be concluded that an optimal age to start breeding was three years of age. It was seen that females starting to breed at age three produced twice as many chicks over a lifetime, compared to females starting to breed at age one. This would suggest that there would be almost no goshawks breeding at age one, but that is not the case. As many as 41.9% started to breed at their first year of life, and only some 13.5% began breeding at their third year, although this would be more beneficial for fitness. When the territory quality is taken into consideration the data reveals that goshawks breeding in good territory reach a relatively high fitness even if they start to breed early. On the other hand females starting their breeding career early in bad territory shows almost no fitness. My personal opinion on why they start to breed early in life, is that they are ready to gamble. The goshawk doesn’t know weather it will be alive or not next year, so instead of waiting for a proper occasion it takes a chance. Reasoning that it would be better to have one or two offspring than none at all. And as this trait is expressed by a substantially higher amount of goshawks than the trait of proper timing, it is probably sustainable.

 

The theory supported by the paper as to why late-breeders show a higher fitness is the constraint hypothesis. Stating that as the females grow older they become more efficient hunters, thus being able to care for more chicks. What is not discussed in the report is what role the male plays in the upbringing of the chicks, as goshawks form pairs. Where the male is responsible for the acquiring of food when the chicks are young, and only as the chicks grow older the female start to participate in the daily hunt. But I theorize that the male also becomes a better hunter over the years, thus being able to supply better for the young, and thereby increasing the number fledged chicks. This process of learning as the years go by can be studied among other species on our planet. Just think of how the young leopard starts of by trying to catch squirrels and end up killing large antelopes. Or how you started working on the summer holidays for a few bob to the hopefully better wage of today. Skills acquired over the years are definitely useful in the attempt to bring up young, no matter if you are goshawk, leopard or human.  

 

References

 

http://en.wikipedia.org/wiki/Goshawk - last visited 2007-04-16

http://sv.wikipedia.org/wiki/Duvh%C3%B6k - last visited 2007-04-16

http://www.jagareforbundet.se/viltvetande/ovrigaarter/duvhok.asp - last visited 2007-04-16

Catry, P. & Furness, R.W. (1999) The influence of adult age on territorial attendance by breeding Great Skuas (Catharactaskua): an experimental study. Journal of Avian Biology, 30, 399–406.

Hamann, J. & Cooke, F. (1987) Age effects on clutch size and laying dates of individual female Lesser Snow Geese (Anse rcaerulescens). Ibis, 129, 527–532.

Krüger, O. (2005) Age at first breeding and fitness in goshawk Accipiter gentilis. Journal of Animal Ecology 74, 266–273

MacLean, A.A.E. (1986) Age-specific foraging ability and the evolution of deferred breeding in three species of gulls. Wilson Bulletin, 98, 267–279.

Nol, E. & Smith, J.N.M. (1987) Effects of age and breeding experience on the seasonal     reproductive success in the Song Sparrow. Journal of Animal Ecology, 56, 301–313.

Pettersson, G. (1984), Europas rovfåglar, Bra Böcker, Höganäs

Williams, G.C. (1966) Adaptation and Natural Selection. Princeton University Press, Princeton.

Wooller, R.D., Bradley, J.S., Skira, I.J. & Serventy, D.L. (1990) Reproductive success of Short-tailed shearwaters (Puffinus tenuirostris) in relation to their age and breeding experience. Journal of Animal Ecology, 59, 161–170.