Breeding

Golden Orioles have bred in the poplar plantations of the Fens of Cambridgeshire, Suffolk and Norfolk since the late 1960s. Breeding was first proven in the Suffolk Fens in 1967 and numbers increased gradually to a peak of about 30 pairs in the area in 1979. Though tree felling at the main plantation site caused birds to disperse across a larger number of sites, birds continue to breed in the area, annually constituting almost the entire British breeding population. The highest number of breeding pairs confirmed in the Fenland basin was 15 pairs in 1991, when up to 21 pairs nested in the area, but in 1994, nine pairs were confirmed and there may have been up to 33 pairs breeding in the area. Numbers have fallen considerably in recent years. The birds have been the subject of detailed study by the Golden Oriole Group and Dagley (1994) and Milwright (1998) summarise a wealth of information gathered by the Group on the birds' ecology and breeding biology in Fenland. The mean date of the arrival of males is 14 May, that of females 21 May. In recent years the majority of pairs have nested in poplar plantations and shelter belts, usually close to rivers and drains, with 94% of nests in the poplars themselves. Three or four eggs are laid in late May and early June (the mean first egg date for Fenland being 1 June) and the young fledge in late June and early July. It is thought that few pairs, if any, lay a second clutch if the first was successful. Adults forage for insects, principally caterpillars and adult moths, in the woodland canopy. Adults and young near to fledging may also take fruits such as cherries. The orioles show a strong preference for nesting in the larger-leaved, early leafing varieties of poplar which develop a spacious, open canopy with a high leaf density. These varieties may provide better cover for nesting birds than the smaller-leaved, later-leafing varieties favoured by foraging birds. Though the birds may forage in young plantations, they generally do not nest in them until the wood is at least ten years old (Milwright 1998).

It must be recognised that Golden Oriole breeds at the northwesternmost extreme of its global range in England. Breeding success is low and is adversely affected by poor weather in June and three times fewer young fledge when the weather is cool and wet (Dagley 1994). The numbers are likely to be maintained, at least to some extent, by immigration from the near-continent (Milwright 1998). The future for Golden Oriole in England thus remains uncertain. Much depends on how global warming affects the bird, not only in Fenland but also in its sub-Saharan wintering grounds and at staging areas between the two.

Breeding Statistics:

Males arrive in mid-May and females arrive a week later
Nest building starts about five days after female arrives
Mean date for first egg laying is aournd 2 June
Mean clutch size is 3.53
Observed incubation periods were 18-19 days
Mean hatching date is 18 June, and the mean brood size at seven days was 2.60
Nestling periods were from 13 to 15 days, and the mean fledging date was 3 July
Failed clutches may be replaced. No evidence was found of genuine second broods
Estimated mean productivity of all nests including failures is 1.4 - 2.0 fledglings per nest Greatest cause of failure is bad weather in June, and nests produced three times as many fledged young in warm dry Junes as in cool wet ones
Most nests are located in poplars
The mean height of the nests in poplars is about 7m below the top of the tree, irrespective of the height of the tree.
Most nests are in the north, northeast or east sectors of the tree, and 80% were within two trees of the edges of the woodland
Breeding densities are generally at or below 0.25 pairs/km2, but up to 2.25 pairs/km2.

Numbers of occupied UK sites & numbers of breeding pairs 1973-2000:

The first figure in the above table refers to numbers of confirmed pairs, the latter to the maximum number of breeding pairs (ar least birds present in suitable habitat in the breeding season).

Reference: Golden Orioles in East Anglia and their conservation. Jez Dagley. British Birds 1994.
Breeding biology and origin of Golden Orioles in Britain. Digby Milwright Bird Study (1998) 45, 320-330

Please note: Golden Oriole is a Schedule 1 listed species, afforded special protected under the Wildlife & Countryside Act 1981. Deliberate disturbance at a know nest site is an offence without a licence from English Nature.