The Darwin Pigeon loft was built by local Downe Carpenter John Lewis. It was based on designs maybe seen in pigeon books of the time,It was hexagonal with a high pitched roof, light coming from openings in the pitch, otherwise known as a lantern, having no windows other than a pop-hole opening into the aviary.
 I would have thought this too dark ,perhaps there was a wire door fitted at the entrance door to let in further light and fresh air, if not it would soon become unpleasant for both Darwin and the pigeons. Breeding cages arranged all around the walls were 4 high and being 3 foot long, 18 inches high, 20 inches deep.
In the loft used for the film "Creation" the cages all had dowelling fronts but maybe only two rows would have been like this, the rest more open, bearing in mind large birds such as English Pouters, Scandaroons and Runts would not be able to access this type of cage. The dowel fronted cages would offer the smaller breeds such as Tumblers and Rollers a degree of safety while rearing young. On the front of the loft was a large aviary, 9 foot high, where the pigeons could be exercised or their habits studied by Darwin such as the mating ritual, bathing and sunbathing.
 
I have included other Victorian loft designs on this page just to give an idea of different styles bearing in mind fanciers would have varied from the well to do to the ordinary working chap in a small house or attic bedroom.
Larger octagonal flint and brick built dovecotes can be seen at Baliffscourt, Clipping, West Sussex New Timber Place, Hassocks, West Sussex.and West Dean Village East Sussex click to view Pictures

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Darwin loft as used in the feature film 'Creation' due for release September 2009, an adaptation of Randal Keynes' book Annie's Box.
Photo courtesy Randal Keynes

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Attic room trap, a pulley system could be used to catch stray birds.

Darwin in Pigeon loft scene

Charles Darwin studies his pigeons from film trailer 'Creation'
Photo courtesy Icon Film Distribution

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Manufactured lean to loft maybe only 6 feet high, 8 feet wide could be placed in a small yard or garden.

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Basic flying loft possibly 10 feet tall to deter cats. Nestboxes would be towards the back on both sides. A ladder would be incorporated to inspect the birds.

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Victorian fancier Mr Wallace's well designed and spacious lofts and aviaries for Carriers, circa 1875.

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This design has plenty of perches around the walls, this allows the hens to rest in the day when the cock bird is sitting on the nest. As the youngsters become independent they move from the lower to higher perches. The horizontal planks protect the birds below from soiling.
The matching/weaning pens [on back wall ] are invaluable to pair unmated pigeons and for weaning 3-4 week old chicks in safety away from aggressive adult cock birds.
Ground level has main nestboxes with lift up inspection lids, a type often used for chicken breeding, the angle stops pairs setting up home on top.

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Original Victorian Welsh valleys loft and aviary displayed at The Museum of Welsh Life, St. Fagans, South Wales.