A READER has contacted me by e-mail to discuss a bird she has welcomed to her garden. It comes regularly to be fed. Her visitor is a blackbird, but it has a white tail feather and she wonders how this might have arisen.

The most likely answer, other than that the bird has dipped its tail into a pot of white paint, is that it is partially albino. Blackbirds are prone to this, often sporting patches of white especially in the wing feathers and tails. From time to time, a completely white blackbird will make an appearance and if it is a true albino, it will have a pink beak, legs and eyes.

The condition affects a variety of creatures. I've even heard of an albino lobster, while humans can also suffer from the condition, but the laws of nature often dictate that a completely albino animal has a very difficult time coping with its own kind. Years ago, I heard of a pure albino red squirrel which was frequently attacked and driven away by its contemporaries, and I think the same can happen to an albino blackbird - it will be regarded as some kind of freak. This does not seem to happen if the bird is only a small patch of white upon it.

Albinism is caused by an absence or deficiency in the body, of the dark pigment known as melanin. The effect in humans is that the hair is white or very light coloured, the skin is white, and the eyes either pink, yellow or very pale blue.

Though the condition is not common in humans, it can occur among ethnic races as well as in other living creatures, both domestic and wild. One result in humans is very poor and delicate eyesight, often requiring dark glasses and specially-fitted coloured lenses. Conversely, there is a condition known as melanism which usually manifests itself in animals, and this produces an abnormal darkness of colour in the skin and hair.

Cold Harbour

Another reader living near Thirsk has raised an interesting question about the name Cold Harbour which often appears as the name of a farm. As many such farms are deep inland, well away from rivers and the coastline, it does seem somewhat unusual.

In puzzling over this, I am indebted to a book called Wade's Causeway by Raymond Hayes and J G Rutter which was published in 1964 by Scarborough archaeological and historical society. The book is a comprehensive account of the incredible Roman road which runs across Wheeldale Moor, near Goathland, in the North York moors. That road has earned the nickname Wade's Causeway, and it is one of our best-surviving examples of a Roman road.

The authors make the point that the name Cold Harbour was once thought to appear on or very close to the route of former Roman roads, adding that the name occurs about 300 times in England. There are 11 Cold Harbours in Yorkshire alone, but research by the authors does not substantiate the claim that Cold Harbour is associated particularly with Roman roads.

In addition to its use as a farm name, it also appears as Cold Harbour Lane and Coldharbour Gate, with Coldharbour sometimes appearing as one word and sometimes as two. One expert believes the name derives from a place of shelter used by travellers as they crossed the countryside in times past, with a London Coldharbour dating no earlier than the 14th century. This coincides with another belief that the name originates from a roadside building of some kind, probably one used as a form of shelter in medieval times, but it does seem that its reputed association with Roman roads has not been proved.

Coldharbour Gate is on the North York moors near Lease Rigg and this might refer to the gate of a former deer park which is known to have existed in the 15th century, but probably dating to Norman times.

It seems there is a German version of the name - Kalteherburg - which may be associated with an earlier (1317) version of our name, ie Caldherbergh, though another theory is that Cold Harbour is just one of those odd names given to farms for no apparent reason, such as High Paradise, Mount Pleasant and Bumper Castle.

Bonomi

Yet another correspondent has written from Leeds to ponder the work of an architect with the wonderful name of Ignatius Bonomi. One of his creations was a bridge across the River Skerne, near Darlington, and this has become famous as the Five Pound Bridge, not because that was the cost of building it, but because that bridge now appears on all our £5 notes. It is one of Darlington's claims to fame.

My correspondent wonders if Mr Bonomi produced any other worthy structures, adding that this architect also designed Eggleston Hall between Barnard Castle and Middleton-in-Teesdale. Built for the Hutchinson family, this is an early 19th century stone house with the ruins of a former church in the grounds.

My own sources show that Ignatius Bonomi was very active throughout County Durham during the early years of the 19th century. For example, in 1832, he re-built the Lumley chapel in the church at Chester-le-Street and in 1841 he also worked on a building on the cast side of the parish church in Crook. He built the splendid Burn Hall at Croxdale in 1821, once the home of the famous Salvin family who retained their Catholic faith in spite of the Reformation; the hall later became a Catholic seminary.

In 1834, Bonomi built Windlestone Hall at Kirk Merrington. This was for the Eden family and replaced an earlier house. It is known for its colonnade of coupled Tuscan columns, and nine years later, in 1843, Bonomi built the church at Ferryhill. At the far end of the 15th century Bridgeford Bridge, which spans the River Wear near Lambton Park, there is a classical arch built by Bonomi which forms the entrance to the park. Dinsdale Park is another mansion designed by Bonomi in 1829 and in the previous year he built the church at Winlaton.

Another of his successes was Wynyard Park near Hartlepool, home of the Marquis of Londonderry. The present house was begun in 1822, the architect being Philip Wyatt, but in 1841 it was gutted by fire and rebuilt under the supervision of Ignatius Bonomi.

I thank my Leeds correspondent for reminding me of the work of this busy County Durham architect.

Boscobel

My heavy mailbag at this time is due to a backlog which has grown during my recent absence in Italy, and another correspondent, this time living in Glaisdale in the Esk Valley near Whitby, points out that my reference to the village of Biscobell (D&S May 25) should have read Boscobel. My correspondent used to live a few miles from Boscobel - readers may remember that this was where King Charles II concealed himself in an oak tree after his defeat at the Battle of Worcester in 1651.

It was this action, followed by the King's triumphant return to London on May 29, 1660, which led to the celebration of Royal Oak Day.

The house in which that famous oak tree grew is still in existence, complete with a priest hole, and there is still an oak tree in the grounds. It is not the actual tree which saved Charles, but is believed to be one which has grown from an acorn produced by that famous oak.

And finally, our wagtails seem to have left our garden but whether or not they produced any offspring is not known. However, a pair of nesting bluetits have made use of our nestbox and have produced a brood. They are now being fed by the parents and chirp anxiously each time we open the garage door, but by the time these notes appear in print, I think they will have flown the nest