European Magpie (Pica pica)
As a member of the corvid family, magpies (the European kind, in any case) are extremely intelligent creatures. Recently scientists testing magpies for self-awareness were amazed at the results of the 'mirror test' - a magpie was placed in a room with a mirror. The bird had never seen a mirror before, and was introduced to its reflection after having a small sticker applied to the underside of its beak. The magpie saw its reflection, recognised itself, saw that there was something wrong with its appearance (the sticker) and then attempted to remove the sticker. Bear in mind that this bird has never seen itself before. Ever.
The magpie did not recognise the image as another magpie - it recognised that the image was a picture of itself. This same test, when applied to humans, is not usually passed before the individual reaches eighteen months of age. Dolphins, humans and great apes are some of the only other species known to pass the mirror test. Magpies are the only non-mammal to ever have passed the mirror test without training.
Yet despite this recognition of potential self-awareness that suggests a shocking level of intelligence, magpies are not protected. They are hindered by their bad reputation in folklore and by public ignorance. Recently UK farmers tried to push for a magpie cull in response to the declining songbird population, and the motion was only prevented when the RSPB (Royal Society for the Protection of Birds) conducted an official study that proved that magpies do not have an overall negative effect on the songbird population - and even if they did, their impact would be by far eclipsed by the impact of domestic cats.
Magpies are viewed in mythology as tricksters and creatures of magic. Their tendancy to collect or 'steal' shiny objects also contributes to this. The traditional folk rhyme that relates to magpies is as follows:
One for sorrow,
Two for joy,
Three for a girl,
Four for a boy,
Five for silver,
Six for gold,
Seven for a secret never to be told.
This version is the one I have grown up with. I am a UK native from the rural Derbyshire Peak District area, so bear that in mind if what I write here does not conform to the version of the rhyme that you are familiar with.
Referring back to the mirror test and the exclusive club of species that have been known to pass it, I pose the question - if magpies can exhibit the same kind of self-awareness as dolphins and human children can, why is it still acceptable for them to be killed? The general public hate the idea of killing dolphins - just think of all that dolphin-friendly tuna - and yet magpies are shunned where dolphins are celebrated.
I decided on Sherlock being a European magpie after much thought, and after reading the above I hope you agree that it is very fitting. Intelligent and disliked? Eerie? Even the colouration.
Magpies mate for life.
Eurasian buzzard (Buteo buteo)
As the most widespread and common bird of prey in the British Isles, the buzzard is a reasonably large bird that is often mistaken for an eagle. They are formidable hunters that drop down on their prey from either a perch or a slow hover, and during the breeding season they are known for their spectacular aerial displays. They are extremely adaptable and come in a variety of colours, and pairs can have many different nest sites. Buzzards thrive in warm climates and have a reputation of being very territorial.
I decided on John being a buzzard because they look rather unassuming, with their brown plumage, rounded wings and brown eyes. They aren't exotic, and are pretty commonplace - but just like John this is only at first glance. Underneath their ordinary appearance they are skilled hunters that mate for life and make excellent parents.
Raven (Corvus corax)
As a member of the corvid family ravens, like magpies, are extremely intelligent. However, they have been tested and, while they exhibit a level of problem-solving and learning ability that surpasses chimpanzees, they have not passed the mirror test (to the surprise of a number of scientists).
A group of ravens is called an 'unkindness,' which really says everything you need to know about what mythology thinks of them. They are known for following fishermen/hunters and stealing part of what they catch. They also do this to wolves. Ravens also use other species on a regular basis for tasks that they are physically incapable of. For instance, upon coming across a large corpse they purposefully attract large predators to the site to open up the carcass for them. There are numerous accounts of ravens 'pranking' other animals - ravens in Yellowknife have been known to perch on the roofs of supermarkets and push snow onto the people who walk below. They have also been seen charging sleeping wolves to scare them.
I decided on Mycroft being a raven partly because of the intelligence, but also because of the sheer weight of mythology behind them. Ravens are regarded in old legends as harbringers of death, and considering Mycroft's role in the show I felt that my choice was pretty darn well justified.
Splendid Astrapia (Astrapia splendidissima)
A little-known bird whose females are small, quiet and dull in appearance, the splendid astrapia is in fact a bird of paradise. A fact that I only discovered through my research is that birds of paradise aren't all about looks - their closest relatives are, amazingly, crows. This means that they are regarded as a 'sibling' group to the corvids, that same group of birds who are renowned for their intelligence.
Molly, to me, is a wonderful person. She is quiet and unassuming, but the truth of it is that she is spectacular. She is clever and heroic, and I love her to bits. As such I felt that a female bird of paradise was the perfect choice - she might not look spectacular, but she is. You just have to be clever enough to see it.
Marabou stork (Leptoptilos crumeniferous)
This African giant is also known as the 'Undertaker Bird' and eats vast amounts of carrion. Their diet includes all sorts of creatures, including other birds, and they can be extremely aggressive. There are recorded incidents of marabous killing children.
They aren't really known for their intelligence, but I wanted Moriarty to be unnerving, and the wings of a child killer are certainly that.
Scarlet macaw (Ara macao)
I chose a macaw for Irene for some pretty obvious reasons. As outlandish, extravagantly 'dressed' masters of communication, what better to represent a woman who makes her living by collecting information for blackmail? The intelligence factor also played a part.
Gyrfalcon (Falco rusticolus)
The gyrfalcon is the largest of all falcons, and it is a straightforward kind of bird. Its flight is fast and low, and when stooping for prey it first flies up and then drops straight down. As the largest falcon it has always been prized by falconers - in some places it used to be that only kings were allowed to hunt with gyrfalcons.
I decided on Lestrade being a gyrfalcon because of how straightforwards they are. They aren't as powerful as a big hawk or an eagle, but they are still skilled hunters that go straight for the kill. They are stubborn and strong, and while they lack the intelligence of the corvids they are still spectacular in their own way.
I feel that the falcon represents Lestrade's single-minded desire for justice as part of his policework, whereas Sherlock is attracted to crime by the 'shininess' of the puzzles (thank you to the commenter who allowed me to put this thought into articulate terms).
European robin (Erithacus rubecula)
Crowned the national bird of the UK in 1960, the robin is perhaps the most iconic of all British wildlife (to UK nationals, at least). A common sight in gardens throughout the four countries, this bird's cheeky attitude and territorial behaviour have endeared it to anyone with even a little bit of interest in gardening. Robins are known to follow people around their gardens, or to even approach people. This appears to be unique to the UK, as during travels on the continent I have never seen European robins as unafraid of people as they are here in the UK. This is probably due to how loved this little bird is in the UK.
I've always thought that Sally appears to feel threatened by Sherlock's presence in Scotland Yard, and the robin's territorial attitude and habit of squaring up to larger birds felt really in tune with this. That's why I chose it.
Harpy eagle (Harpia harpyja)
One of the largest and most powerful eagles of all, the harpy eagle hunts and kills many things but is especially well known for how it hunts monkeys. They chase their prey relentlessly, and can have talons as long as the claws on a bear.
This bird is a truly magnificent hunter, and so I felt that it was very well suited to the character of Sebastian Moran. Seeing it attack a camera crew who were trying to film it for a documentary helped, too.