At the sound of footsteps, Bourbon the grey seal raises her head and swivels her eyes – great ink-filled globes – in the direction of the approaching voices.

Abandoned and hungry, the one-year old seal was found struggling on a beach several days earlier. She is one of four young seals being cared for at the Scottish SPCA's new national wildlife rescue centre at Fishcross, near Alloa. She is still on high alert in her strange new environment but is showing signs of improvement.

"She could barely lift her head a couple of days ago," says wildlife assistant Claire Stainfield, who runs the seal unit. "We give them fluids when they first come in because they are dehydrated. She has worms too which has given her a rather snotty nose."

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As with many of the reports of seals in trouble, staff were alerted by a dog-walker who called the Scottish SPCA helpline.

"One of our drivers went out at 11pm to the beach where she'd been lying for about three days. A jogger came by and together they managed to get her into the crate so they could bring her here.

"You always know that a seal is in danger if you can walk right up to it. Normally, they'll see you before you see them and they'll hear you and smell you and be straight back into the water. If they are babies then they might have come in to shore to rest or have got lost or been swept away."

Bourbon is clearly on the mend as she has started to eat and her weight has gone up to 27kg; the average weight for a healthy one-year-old is 50-60kg.

In the next two pens are the "babies", Oreo and Ginger, two very young, very thin little harbour seal pups. Grey seals and harbour, or common, seals are both found around the Scottish coastline. To the untrained eye they are hard to differentiate but greys are larger with flatter noses.

Then there is Tunnock, the longest resident, who has been here since the beginning of June. "He came in just one or two days old having been found in East Lothian. He still had his umbilical cord attached and it had got slightly infected so he had got a bit run-down. We put him on antibiotics and he just bounced back."

Tunnock is now feeding himself and just needs to put on a little more weight before he can be released. "My job is to get them fit and ready and to feed for themselves, so I won't act like a surrogate mum and sit and stroke them or anything like that," says Stainfield.

"We try not to interact with them too much. The harbour seals are a little easier to tame because that is their nature – they are a lot more trusting, a lot less aggressive. The grey seals have got a bit more attitude to them – they are more feisty and they don't want anything to do with you from the minute they come in. The minute they are self-feeding, that's them happy."

Stainfield, 23, who has a degree in marine biology from St Andrews, joined the Scottish SPCA last August. "I used to work with seals in aquariums and this seemed like the natural progression. It's so much more rewarding to do something to help them."

When they first arrive, seals are placed in a dry pen until they get their bearings before being moved to a wet pen and from there to the outdoor pools where several seals are put together. "In the outdoor pool, they learn to compete with other seals, they build up their swimming muscles and get ready to go."

The release point is Aberdour beach in Fife. "It's amazing. I try to go on as many releases as possible," says Stainfield. "I think it's the same for all the staff. It is what you are doing the job for. It is tough when you look after an animal for four months and it doesn't make it. You do get hard times and days when nothing seems to be going right, but you get through it and you get the odd one that recovers against the odds."

The seal unit is one of several single-storey buildings which make up the £3.5 million rescue centre, which opened in April, and occupies a tranquil spot near the foot of the Ochil hills.

Built to replace the old centre in Fife, it is equipped with veterinary facilities, a stable block for deer casualties, aviaries and wild mammal enclosures. The location, in the heart of the central belt, means it is accessible from all directions.

The move from the former premises near Dunfermline was prompted by a 70% increase in the number of wild animals being treated over a four-year period; a rise which is due to increased public awareness of the rescue service, coupled with more staff to collect the animals. The centre has two managers and nine wildlife assistants and last year 3782 wildlife casualties were admitted with 2504 of these animals rehabilitated and released.

"It is early days, but we designed it in such a way that it would be able to cope with more animals and would be able to give them better facilities and enable us to offer them better care," explains centre manager Colin Seddon. One of the changes is that once the animals arrive at the centre, they will be cared for here until, hopefully, their release. Previously animals, particularly otter, fox and badger cubs and deer, would be moved between centres.

"In the old centre, we had no facilities for keeping them long term so all we could do then was stabilise them, wean them if they were on the bottle, and then send them to a private centre, not an Scottish SPCA one. That was one of the main reasons for us moving here, so that as a national organisation we can do the whole job right the way through so there is no wild animal we are not equipped for, other than whales and dolphins.

"We have dealt with everything from eagles to mice and white-tailed eagles. Pine martens are probably one of the most unusual things I have seen."

The original rescue centre had been designed to deal with oiled birds but latterly cared for all kinds of wildlife. This custom-build site is able to deal with up to 1000 oiled bird casualties at any one time. To put this in perspective, after the massive BP Gulf oil spill in 2010 more than 2300 oiled birds were collected along the US coastline.

Birds still make up a large proportion of the animals treated. In one of the rooms in the birds section, a house marten the size of an index finger is blindly opening its tiny beak to receive a tweezer-full of food. Lorraine Gow, head of the birds section, is gingerly placing minced beef mixed with energy formula into the bird's pink mouth.

Many of the house martens have suffered fractures when their nests have fallen or been knocked to the ground from the eaves of properties. Others are malnourished because heavy rainfall has meant the parents were unable to find insects to feed them.

Tiny week-old house martens cluster together for warmth. "With the smaller ones, they need you to be like mum," says Gow, as she picks one up and uses her fingernail to prise open its beak. "With the older ones, if they show signs of self-feeding, we recommend stopping hand-feeding." At this age birds need fed every hour, which means Gow, who is looking after around 170 birds, is on a constant feeding circuit. In an incubator, a blackbird chick, all fluffy light brown feathers flecked with black, opens his beak in expectation of the upcoming feed. Only three weeks old, he was brought in after he was "catted", a fate which many of the birds have suffered.

"I always think of it as being like M*A*S*H or ER because the casualties come in and you have to deal with them. You don't know what is coming through the door next, what disaster has happened."

The next incubator is the temporary home to a four-day-old herring gull chick found in St Andrews. "He's probably been abandoned or just wandered off," says Gow.

"People always go on about them being a pest species and say that herring gulls are disgusting but they live for 35 years, that's the average lifespan, so they are right up there with parrots and they are so intelligent. In fact, a lot of birds are very intelligent. People don't realise that about your wee blackbird and sparrow, everyone just sees them as things that fly around the garden, but when you really look into their eye, you can tell."

After leaving college Gow, 41, worked as a graphic design artist for eight years. Passionate about animals, she hand-reared tits when she was a teenager and began volunteering with the Scottish SPCA in 2002. The following year she was offered a staff job. She loves her new career.

"Obviously the pay is different but the actual work is so much better. I've always loved animals and I've always rescued things off the road. I see animals as important as people.

"We are not working for profit. When I am here, I am working for the animals. I just want to see them get released which makes it even harder because when things don't work out you can get really disheartened. If you started to worry about things, you'd end up getting run-down."

She says that many of the staff hand-rear some of the animals at home in the evenings and weekends to ensure that they are fed. "There have been a few times where you almost burn out over the summertime when there are lots of people on holiday. You're working at home and on your day off and you can feel like you are just about to frazzle."

Colin Seddon explains that having a specific member of staff matched with a particular animal limits the amount of human contact the animal has and helps ensure it is not tamed and will fare better on release. It does mean, however, that the responsibilities of staff don't end when they walk out of the front gate.

While losing an animal is always hard, bringing one back from the brink of death is what everyone here lives for.

"It's the best feeling in the world," says Gow. "All these birds and animals in here, they would all be dead now if it wasn't for people phoning up and inspectors and drivers going out and picking them up and bringing them in."

Sure enough back at reception, Bob Ward, one of the animal rescue officers who collects the casualties, appears bearing a cardboard box containing the latest arrival. While occasionally people turn up at the centre with an injured animal, most are brought in by the officers in response to calls. Inside is a young hedgehog which has been spotted in a garden in Dunfermline. As a nocturnal animal, the fact that it was in clear view during the day is a sign that all is not well.

Wildlife assistant Stuart Louch takes the hog into a side room to check it in. Before retrieving the animal from its box he snaps on a pair of gloves as ringworm is always a danger when handling hedgehogs. He gives the animal a once over for mites and maggots. If it is found to have either it would need to be isolated from the 13 other hogs which are already here. He also does a check for any obvious injuries which will be treated by the vet, who is onsite one day a week.

"They often suffer strimmer injuries because they will be in the long grass and they get it right across the back. You do see some gruesome injuries. If a fly gets near the wound, there can be maggots. You put it to sleep because there would just be more suffering for it."

The animal doesn't appear to have any wounds so Louch weighs him and rubs its spines in one direction, essentially giving him a massage to de-stress him, before placing him in his own incubator.

Another common complaint among hedgehogs is salmonella, which is deadly in the majority. "Before, when we didn't have the space, they were kept together and we found that it was a problem. Here, because we've got more space, as soon as they are weaned we are separating them. Some 25% of the natural population carry salmonella and are not affected by it. The other 75% are. If in a litter of four, one has it and the other three are interacting with it, they will contract it."

During his stay the little hog will be fed goats' milk before moving on to chicken-flavour cat food and eventually dried insects will be added to the mix. Most animals are released when they reach 500g but as many arrive weighing a paltry 90g, that can take a month or two.

Over in the corner, Squirrelette, a lively seven-week-old red squirrel, springs to life when the cloth is removed from his cage. He was found by a woman at the foot of a beech tree in her garden. Her dog had come across it and run away. He was too young to be independent and had probably fallen from a tree or been dropped by a predator. When he arrived he had a damaged jaw which has now healed. As he has been putting on weight in the last few weeks, he'll be moved to an outside aviary within the week before being released at a site in Perthshire.

With the new arrival settling into his abode, Louch, who works mainly with the mammals, leads the way to an outdoor enclosure which contains four pine martens. A glimpse through the peep hole in the wooden door reveals a flash of auburn fur and as Louch edges the door open a little black nose pokes out.

Nearby, a wild rabbit in one of the enclosures stops in his tracks. His head is slightly tilted, which suggests he has been hit by a car. Louch is not sure about his future. "Sometimes when you look at an animal, physically it is OK but you can tell it's just not there," he says. "You have to ask, is this animal going to make it in the wild? If the answer is no, then you might as well end it now. If the answer is yes, then you go for it and if it's maybe, you'll keep asking and keep setting goals for it and hopefully you'll see some progression."

For the majority of animals who pass through the doors of the rescue centre, the ending is a hopeful one. As we go to press, word comes in that Tunnock and Bourbon are being released for their second chance at life in the wild. n

Scottish SPCA helpline: 03000 999 999.