All sorts of discoveries can stop work on a building site: bodies (ancient or recent); archaeological artefacts; or contaminants such as asbestos or heavy metals. This is one reason why you might find anthropologists and archaeologists working side by side with builders and engineers.

But one of the most common reasons for delays is the discovery of a legally protected species, such as bats, dormice or great crested newts (pictured). Ian Jones, a contracts manager at Durkan, a construction company, recently worked on a housing project delayed by the discovery of slowworms - a type of legless lizard. “You always do a sweep of the site before construction starts,” he says. “We had guys with strimmers taking away the grass, looking under rocks and boulders and following [them was] a young lady from Entec looking for any sort of wildlife that might be around.”

No sign of protected wildlife was found so building began. “Then, off to the side of the site, we had to clear an area to make a footpath. As we started to work we found our first sign of slowworms - a baby one.” As soon as this happened, Jones had a duty to stop any work that could harm or disturb them until they could be rehomed. This was problematic because they had found the creatures in their hibernation period, which meant they could not be moved for several months, after which Entec's consultants spent a week going over the arearemoving every slowworm.

Still, Jones was relatively lucky in that the reptiles were found on only part of the site, meaning that work could continue in other places. Had they been found on the whole site, the project would have had to shut down entirely until they could be safely removed.

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While this discovery was made during construction, many delays occur much earlier in the process. Planning permission is often dependent on developers showing that no protected animals, plants or buildings will be adversely affected by construction.

One of the main tasks occupying Entec's 50 or so ecologists is preparing these initial reports, says Richard Knightbridge, the firm's technical director of ecology. “We go on to a site and do some initial ecological surveys, which look at the potential for there being legally protected species present,” he says. “We also look at other ecological constraints; for example, uncommon or valuable habitats such as ancient woodland or ponds, or notable species that aren't protected but which are an important consideration for development.”

If necessary, the development can be modified to avoid or reduce negative effects, thus helping to gain planning consent. Species are moved to another site only if it cannot be avoided; this may in itself require consent from a statutory body such as Natural England. This is a very hands-on job; ecologists need to know how to identify habitats, signs that an animal might live in the area and the animals themselves - it's quite different from looking at specimens in a lab, Knightbridge says. Most have a biology-related first degree with a masters or PhD in ecology, conservation or a related field.

Inland, a property development company, buys brownfield sites (used for industrial or commercial purposes in the past) for redevelopment. This means getting appropriate planning permission by going through a range of steps, including the ecological surveys described above. The longer the delay between buying the site and obtaining planning consent, the more it costs his company, says Paul Brett, Inland's land director.

“We have a £50 million redevelopment in Hampshire where we have been delayed for a year because of a bird called the Dartford warbler.” As a result, the site is in a Special Protection Area, which means that developers must undertake detailed lobbying and negotiation with local authorities before planning permission is given. Usually this means the developer giving money to the council so that it can improve another wild habitat in return for development being allowed on the first site.

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