Manx Shearwater - 2012 - research

Hormones dictate when youngsters fly the nest

Published Thursday 5 July 2012

Seabirds feed their young less as they reach an age to fly the nest, but it’s hormones that actually control when the chicks leave home, according to new research from the University of Leeds.

The study – published in Behavioural Ecology - aimed to pinpoint the main trigger which causes chicks to leave the nest and embark on an independent life, a process known as fledging.

While studying a colony of Manx Shearwaters (Puffinus puffinus), on the island of Skomer, researchers from the University’s Faculty of Biological Sciences noticed that parent birds seemed to become increasingly insensitive to their chicks’ demands for food as they grew close to fledging. At the same time the chicks showed a marked increase in levels of the hormone corticosterone. However, the team needed to know whether this increase was independent of, or caused by, the reduction in feeding.

They decided to trick the parent birds, by swapping chicks of different ages between nests – which the birds make in burrows in the ground – to see how this affected both parental care and the time chicks took to fledge.

“Manx Shearwaters don’t recognise their own offspring, but will simply go back to the same nest after they’ve gathered food. They have one chick, which makes the interactions between parent and offspring easier to study,” explains lead researcher, Dr Keith Hamer. “We swapped chicks which were between 10 days and two weeks apart in age, to see what impact it would have. We wanted to find out whether parents and chicks were responding to each other’s behaviour, or whether each was acting independently.”

The team discovered that adults reduced their food provisioning after about 60 days of raising a chick, regardless of the chick’s stage of development. Although females more than males will adjust their feeding levels to how much their chicks beg for food, after around 60 days both parents start to ignore their pleas. This held true whether parents were feeding their own chicks, or foster-chicks of different ages.

The surge in corticosterone took place over the final few weeks before chicks fledged at about 70 days of age. This held true even when chicks had been fostered by parents at a different stage of the feeding cycle, so was clearly independent of the parent’s behaviour and any reduction in food. 

“Our findings show that young Manx Shearwaters leave home of their own accord when their corticosterone levels have reached a peak rather than as a result of changes in parental behaviour,” says Dr Hamer. “Both parents and chicks need large energy reserves for their arduous migration across the Atlantic to South and Central America, and parents seem to reduce how much they feed their young simply to protect themselves.”

“Unlike some other bird species, which let their offspring dictate the level of care, seabirds appear to weigh up the cost of a chick fledging underweight against the greater cost of losing the chance to breed again,” he adds. “Manx Shearwaters have a breeding life of around forty years, so parents pay a high cost if they end the season too weak to complete their own migration.”

Image credit: David Boyle

Press release by Univeristy of Leeds

Keith Hamer et al. Parent–offspring conflict during the transition to independence in a pelagic seabird.Behavioural Ecology, 2012 DOI: 10.1093/beheco/ars079

Image credit: David Boyle

Manx Shearwater - 1949

Puffinus puffinus puffinus

Mathew called Skomer "the largest breeding-station . . .in the British Isles," which may be correct.  He believed that a few might nest on Ramsey; the Modern Universal British Traveller, 1779, states that the "Harry-bird" (ie the Shearwater, see Oxford Dictionary 1901) breeds at Ramsey, and there is a good description of it nesting in holes there.  Mathew gives some evidence that they were possibly breeding on St Margaret's Island in 1893, and perhaps also on Caldey; he seems not to have known of the Skokholm colony.  About 25,000 pairs breed on Skomer, and at least 10,000 pairs on Skokholm.  Visits the whole coast at night, from March to August, and often heard calling inland (eg Haverfordwest)

R.M.Lockley, G.C.S.Ingram, H.M.Salmon, 1949, The Birds of Pembrokeshire, The West Wales Field Society

Manx shearwater - 2008 geolocator study

Migration and stopover in a small pelagic seabird, the Manx shearwater Puffinus puffinus: insights from machine learning.

Abstract

The migratory movements of seabirds (especially smaller species) remain poorly understood, despite their role as harvesters of marine ecosystems on a global scale and their potential as indicators of ocean health. Here we report a successful attempt, using miniature archival light loggers (geolocators), to elucidate the migratory behaviour of the Manx shearwater Puffinus puffinus, a small (400 g) Northern Hemisphere breeding procellariform that undertakes a trans-equatorial, trans-Atlantic migration. We provide details of over-wintering areas, of previously unobserved marine stopover behaviour, and the long-distance movements of females during their pre-laying exodus. Using salt-water immersion data from a subset of loggers, we introduce a method of behaviour classification based on Bayesian machine learning techniques. We used both supervised and unsupervised machine learning to classify each bird's daily activity based on simple properties of the immersion data. We show that robust activity states emerge, characteristic of summer feeding, winter feeding and active migration. These can be used to classify probable behaviour throughout the annual cycle, highlighting the likely functional significance of stopovers as refuelling stages.

Full article here

Citation: Guilford, T., Meade, J., Willis, J., Phillips, R. A., Boyle, D., Roberts, S., Collett, M.,Freeman, R. and Perrins, C. M. (2009). Migration and stopover in a small pelagic seabird, the Manx shearwater Puffinus puffinus: insights from machine learning. Proc. R. Soc. Lond. B 276, 1215-1223

Manx Shearwater - 2003-07

The Manx Shearwater colonies of the Pembrokeshire islands total around 50% of the world population, with around 120,000 pairs on Skomer, 45,000 pairs on Skokholm and 4,000 pairs on Ramsey. Because of the importance of this Shearwater (in European and indeed World terms) these island have been designated a Special Protection Area for them (under the EC Directive on the conservation of wild birds (79/409/EEC)

They are very difficult to census accurately and methods have changed over the years, from simple estimates based on counts of birds seen at night, estimates of burrow densities through capture/recapture methods derived from known numbers of ringed birds, through to the present day estimates based on counts of burrows and the responses of adult birds to tape recordings of their calls. Each method has been refined and compared but throughout there has been strong evidence of a continuing slow increase in the populations in the last fifty years, perhaps with an indication of a reverse of this in the early 2000’s.

On Ramsey Island the small population (perhaps 1,500 – 2,000 pairs) has risen rapidly since the Brown Rats were removed in 2000. In 2008 the population was estimated as 4,000 pairs.

Steve Sutcliffe.

Fieldwork 2003-07

Red = breeding confirmed = 3

Total tetrads in which registered = 3

Manx Shearwater - 1994

Numerous breeding summer visitor. Recorded in all months

The Manx Shearwater figures as the emblem of the Dyfed Wildlife Trust, an appropriate choice since internationally important numbers of the bird breed on its island reserves of Skomer (estimated 95,000 pairs in 1971 and 165,000 pairs in 1989) and Skokholm (35,000 pairs estimated in 1973). Small colonies also occupy Middleholm and North Bishop. Large numbers probably bred on Ramsey prior to the invasion of the island by brown rats, which is estimated to have taken place in around 1800 (Saunders, 1986). However, by the end of the nineteenth century numbers had greatly reduced, and Mathew (1894) could only suggest that "a few may breed". No definite breeding records on Ramsey have been traced for subsequent years until R. Pratt's record of 200 pairs in 1975 and the RSPB estimate of 300-400 pairs in 1992. In the past, Manx Shearwaters also bred on Caldey and perhaps St Margaret's Island (Mathew 1894).

Manx Shearwaters return to the breeding colonies in late February and depart from late August, some young not leaving until well into October. They visit the nesting burrows only at night, a stratagem which limits the effects of predation by gulls.

They use all the sea areas around the county for feeding, particularly the southern Celtic Deep, increasing numbers penetrating further northwards as the season progresses. Varying numbers also use the Bristol Channel, sometimes following fish shoals as far upstream as the Severn Bridge. Large processions can be seen passing our headlands each evening as they head for the island colonies, assembling in rafts on the water to the seaward side to await nightfall before moving ashore.

A few remain in local waters throughout the winter but the majority cross the Atlantic to winter off the coast of South America, where they are found mainly between latitiudes 20°S and 30°S until December. Return passage may be by way of the west African coast and certainly through the Bay of Biscay. Some first and second year birds summer off Newfoundland and Nova Scotia while others return to their natal colonies, and also visit colonies elsewhere in the Irish Sea and possibly Scotland. Extreme ringing recoveries include a Skokholm­bred bird that wandered as far as South Australia by the beginning of its second year, and an eight- year old in Norwegian waters during May.

Some fledglings wander inland when they leave the colonies, with onshore gales blowing many into the Cleddau Estuary and beyond, sometimes as far as eastern England. The gales also result in passages of thousands off Strumble Head. Many of these could have drifted downwind into the Irish Sea to beat back out to regain their 'ground' but it is highly likely that some could be Scottish birds passing southwards through the Irish Sea.

Fieldwork 1984-88

Red = breeding confirmed

Orange = breeding probable

Yellow = breeding possible

Donovan J.W. & Rees G.H (1994), Birds of Pembrokeshire

Manx Shearwater - 1970s breeding

Red = breeding confirmed

Orange = breeding probable

Yellow = breeding possible

Manx Shearwater - 1894

Species account from M Mathew, 1894, "The Birds of Pembrokeshire and its islands"

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