What are the limiting factors to expansion?

Ospreys exhibit a low ability to recover lost range, with a natural average spread of less than five kilometres per year over time. In North America, following the huge losses due to pesticide poisoning in the 1950s and 1960s, this was overcome by a series of a translocations involving the hacking of young.

It is well-known in Europe, that male ospreys prefer to breed near their natal site while females may breed anywhere within the population, occasionally including over long distances, even between Sweden to Scotland, northern Germany to France. When a pair of ospreys move to a new locality in an expanding population, rarely more than 50 kilometres from the present breeding range, new pairs join them to establish a loose ‘colony’. If they are not joined by other pairs within a period of several years, the chances are that the pioneering pair will not establish a new ‘colony’ and on the loss of one or both of the original adults, the colonisation attempt fails. The growth of each new ‘colony’ is slow at the outset but, if successful, it rapidly increases and then levels out. In some case it may even decrease.

Despite these old established ‘colonies’ being full, there is still much competition by young potential breeders to join them. An examination of colour-ringed breeding adults in Northern Scotland over many years showed that annual survival was 91%. This means that on average in a ‘colony’ of 10 pairs of ospreys there is only a requirement for two new replacements per annum, yet we regularly record many intruders at nests within these areas. Non-breeding intruders often visit nests containing pairs throughout the nesting season. Sometimes these visits can be surprisingly aggressive and can result in broken eggs or even the eviction of resident adults. Ospreys may even be killed during these fights. Field research has also shown high degrees of interference by intruders which could be causing density-dependent effects.
Age of first breeding

Ospreys normally breed at three years of age and in the early years of population growth, breeding at three or four years is normal. Ospreys can breed at two years but it is very unusual. In established ‘colonies’ more individuals are forced to delay first breeding and this is believed to be linked to birds being intent on breeding within established ‘colonies’, rather than moving to unoccupied regions. For example, the following table demonstrates the delay in breeding.

What is clear is that many ospreys are now not breeding when first mature and capable of doing so. They are waiting until later to find a place within a favoured ‘colony’, usually due to the death or non-return of an older bird at an established nest. Each delayed season for an individual means a 9% chance of dying before the next breeding attempt. This is due to the fact that average annual survival of adults is approximately 90%. A bird failing to establish itself until its fourth, fifth, sixth or seventh year therefore has a greater chance of death before breeding successfully, an additional 9%, 18%, 27% or 36% chance respectively of dying. Females are most productive in the early years of their breeding life, so this failure to start breeding has a double disadvantage. Earlier research in North America showed that the mean age of first breeding in an expanding population in New England was 3.6 years, but in an established population in Chesapeake Bay was 5.7 years (Poole, 1989).

In many areas, the productivity of translocated populations throughout Europe may well be higher than donor populations. So not only do translocations have the potential to enable translocated birds to breed earlier, but also to breed more productively. This has the effect of increasing the overall population more rapidly.

Management techniques for restoring breeding ospreys

Artificial (human-made) nest building

Artificial nest building increases breeding success and productivity, and range expansion. It is most effective in extending the edges of breeding areas and creating intermediate nesting locations between established ‘colonies’

Of course ospreys can build their own nests, but in stable populations and ‘colonies’ most ospreys choose to breed for the first time in an old established nest that is vacant or, ideally, occupied by a bird of the opposite sex which has lost its mate. The natural behaviour of ospreys then is to look for big old nests rather than build their own. If ospreys have been exterminated from a region the lack of ‘old nests’ is a disincentive for ospreys to stop and breed. For that reason, the building of artificial nests is an important management tool for recovery. These man-made nests must be very well built in the best sites, must replicate previous use by ospreys and be regularly monitored and repaired.

The preference to breed in established but saturated ‘colonies’ and to delay the age of breeding suggests that it is not only good management practice to translocate young ospreys to new areas of the extinct range but that it also gives individuals greater opportunity to breed at an earlier age and have greater lifetime reproductive success by breeding in new localities. It also removes some of the potential future intruders from saturated ‘colonies’.

Leaving ‘excess’ young in full old established ‘colonies’ means that at these ‘colonies’ the age of first breeding is delayed, individual lifetime reproduction falls and breeding success declines due to intruder aggression and possibly through competition at fishing sites. There is no doubt that ‘pools’ of non-breeding intruders decrease breeding success within saturated ‘colonies’.

Reintroduction of raptors and other species to areas where the species has become extinct is now an accepted wildlife management practice throughout the world. Osprey reintroduction and translocation was pioneered in North America from the 1970s, and is increasingly being used in Europe.

Translocations, artificial nest building and nest manipulation (supplementing broods of remote pairs with translocated young) are the key to proactive range recovery. ‘Colonies’ reaching their peak are the best sites to provide donor young and because of natal philopatry and delayed breeding age, the removal of young has little effect on local populations. Furthermore, removal of young birds from areas that have reached carrying capacity reduces negative density dependent effects. The translocated birds in new areas may breed from three years (a few at two years) while in full ‘colonies’ first breeding may be delayed past three years to 4 - 7 years. Pioneering and isolated pairs should be enhanced by fostering chicks to ensure maximum brood sizes (as was successfully carried out in Andalusia) and by small translocations, otherwise they may fail. Translocation projects can be reduced in cost and effort as techniques become well proven.

There is a tendency to be very conservative with regard to taking young, under licence, from donor populations for translocation projects. An investigation of the biological facts shows that quite high numbers can be collected. In a ‘stable’ population of say 100 pairs of breeding ospreys, the annual mortality of adults per annum would be 18 individuals (9% mortality) from the 200 adults. Thus each spring there is a requirement for 18 new breeders to join the population. Average annual breeding success for Scottish and northern European populations is approximately 1.35 young per all nesting pairs, which is a total of 135 young reared annually. 60% or more of young ospreys die before reaching breeding age, so from the annual production of young, 54 potential breeders should return. Taking a lower survival rate of 35% rather than 40% this is still 47 birds. Not all young return to their natal area but in large overall populations it is possible to predict that there is a general surplus of 29 – 36 individuals in any area, holding 100 pairs, in order to keep the population stable. Donor populations can therefore provide up to 30 young per 100 pairs for translocation projects without causing reductions.

Osprey translocations have attracted some opposition from conservation bodies and ornithologists but not from the general public. The key objections are, ‘Why not leave it to natural spread?, ‘It will damage the donor population’ and ‘It will be expensive and take away much-needed money from more important conservation’. This last point also involves the fact that the osprey in Europe is presently classed as an IUCN listed species of least concern.

It has been proved that natural spread in some raptors, like the osprey, is very slow, whereas with more mobile species, such as the peregrine falcon, rapid recolonisation of lost range can take place, as with, for example, the return of the peregrine to all of the United Kingdom after the banning of the persistent pesticides in the 1960s and 1970s. Ornithologists have scientifically examined the issue of removing young from donor populations and found that it was difficult to detect any effects on the population of the removal of small numbers of chicks from the population. This has been examined by Dr M. Marquiss (ITE, 1996) for the Rutland project and by Dr P. Whitfield (SNH, 2004) for the Spanish project. Such effects are even more difficult to observe in the field because the ‘colony’ effects described above distort recruitment and breeding success.

It is also a fact that the removal of birds from an area does not remove the birds from the total population but simply redistributes them. Opposition may involve local and national eco-politics, while the claim that osprey projects take money away from more ‘worthy’ conservation is not proven, because donors do not necessarily behave in a way we expect: the iconic osprey may attract donors who do not normally support conservation projects. Experience of osprey translocation projects developed over the past twenty years in Europe (and longer in North America) demonstrates that such projects can be reduced in both cost and effort as techniques become more accepted.

In some parts of Europe – particularly in the UK and Finland – ospreys generate widespread interest among the general public. In some areas they have become vital to the rural economy through increased tourism revenue. For instance a single osprey nest in Cumbia, England is estimated to contribute in excess of £2 million to the local economy every year. Although the impact of newly-established osprey populations will vary between different European countries, the economic benefits of restoring the species to other parts of its former range could be considerable.

The osprey’s future in Europe varies between different regions and therefore the management proposals are therefore different. The aim of this document is to encourage individual countries to review the conservation requirements of ospreys, nationally and as part of an overall European conservation plan.