It is a questioned that has often been posed about our feathered friends.

Now the future of the country's offshore windfarms may just depend on the answer to: "How high do birds fly?"

RSPB Scotland is to bring a legal challenge against the government over plans for four offshore projects in the firths of Forth and Tay, based on its mortality predictions for sea birds.

A total of 335 turbines are planned, but there are concerns abound birds being displaced and hitting the structures, and the impact on migrating species and on birds' commute from breeding sites to feeding areas,

Researchers are being recruited to help answer the question by ministers and quango Marine Scotland to research the effects.

But the Scottish Government is being coy about the total outlay of public funds that is to be spent on the 15-month long project, entitled:'How high do birds fly?" They say they cannot put a price on it because of commercial considerations.

The advert's jargon says it seeks to "potential interactions between offshore renewables and marine wildlife as a matter of priority so that appropriate mitigation can be investigated and applied."

It says Scotland has the potential to generate a quarter of Europe's offshore wind potential, but the "concerns with respect to the potential impact of offshore wind farms on seabirds have been a limiting factor for offshore wind farm projects throughout the UK."

Those most at risk of collision with wind turbines include gannet, kittiwake, herring gull, lesser black-backed gull and great black-backed gull, with terns and skuas.

The tender calls for monitoring by laser range finders, costing £500 per unit, radar and by boat observers, but warns of the risk of errors from "observers tending to underestimate flight heights."

Linda Holt, a spokesman for anti-windfarm campaign group Scotland Against Spin said: "There is considerable uncertainty about the height at which birds fly, and so no one has any idea how many birds massive banks of turbines up to 200m (656ft) in height in Scotland's coastal waters will kill.

"Why on earth wasn't it (the research) carried out years ago as these offshore projects have been in the pipeline for at least 10 years?

"The problem is that once turbines are built and killing birds, the only effective remedy is to demolish them, and no one can see that happening."

A Scottish Government spokeswoman would not comment on the cost

She said: "We can cannot say at the moment how many researchers will be involved in the project or what pay scales they might be on, as we have yet to receive tenders."

She said it would be down to those designing the tenders to decide how many researchers would be needed and how much they could do the work for. She could not put a cost on it for commercial reasons.

Scotland has more turbines than the rest of the UK put together, with almost 5,000 approved.