Two buzzards and two sparrow hawks were found by a dog walker next to the A68, south of Pathhead, after being killed with what is thought to have been industrial grade chemicals.
Police believe the birds may have been thrown from a vehicle as it drove past the stop point and appealed for any witnesses to help.
The incident comes just weeks after the Evening News told how officers were still hunting criminals who killed two peregrine falcons and took the head of one as a trophy.
Pc Hannah Medley, Midlothian wildlife and environmental crime officer, said the birds in the latest incident were discovered on Monday, September 5.
“It appears to be two sparrowhawks and two buzzards which have been dumped, possibly thrown from a car, in a lay-by on the A68. We’re appealing for any witnesses who may have seen anything in the area on that day.
“We’re still waiting to find out what the poison is, but these poisoned baits are often fairly strong and would be well capable of killing a family pet.”
She added: “It’s very unusual for this to happen in Midlothian, and we haven’t really had anything like this recently.
“I’d say to anyone who knows what has happened to contact police. It could possibly be a car speeding off or anything suspicious in the area that didn’t look quite right.”
The incident also came as the Royal Society for the Protection of Birds Scotland prepares to publish its annual report into illegal killings of birds in Scotland last year.
The report includes details of golden eagles and red kites, and a raid on an estate in Sutherland in which 10 kilos of lethal poison were found.
Ian Thomson, investigating officer at RSPB Scotland, said: “This is an absolutely heartless crime and the fact that people are still committing this kind of acts in 2011 is disheartening.
“This case is very unusual, why someone would want to dump these in a lay-by isn’t clear. One of the key things coming from our annual 2010 report into illegal killings of birds is the amount of illegal poisoning that is going on.
“We’ve seen four golden eagles, seven red kites and these are just the ones found.
“More often than not the chemicals that are used have been banned for many years.
“Last year we recovered 10.5 kilos from an estate up in Sutherland, but that was enough to kill the entire bird of prey population of Scotland six times over.”
He added that there was a significant risk of family pets being poisoned by accident.
He said: “When you put a poison bait out in the countryside, whether it’s to poison a fox that’s been bothering your chickens or whatever, it is completely indiscriminate.
“It could be a golden eagle, a red kite, or a pet dog, and one of these days to be perfectly honest a child’s going to touch one of these baits and it could lead to tragic consequences.”