A post-mortem examination on the raptors has confirmed that illegal pesticides were used to kill the birds.
A total of three dead birds were discovered in the Stewartry area in April but the cause of death for the remaining red kite has not yet been established.
It brings the total number of birds poisoned in the region since the beginning of 2018 to four.
In January, a red kite was discovered in Kirkpatrick Durham followed by the discovery of a buzzard in Mossdale in March.
Constable Alan Steel, specialist wildlife crime officer, said: “What we have established is that illegal pesticides have been used to kill four of the birds.
“The pesticides identified have been banned in the UK for many years but, despite this, there would still appear to be those who leave out poisoned bait, whether that is to target crows, foxes, raptors or other wildlife.”
Wildlife officers say the use of poisoned bait is illegal and “totally unacceptable”, and those responsible should understand their unlawful activities have devastating consequences on other wildlife.
Mr Steel added: “We have also carried out a number of land searches in the vicinity of where the birds have been located with a view of trying to locate poisoned bait at these locations.
Anyone with information about the deaths of the birds or knowledge of who may be using illegal pesticides in the Stewartry area should contact Police Scotland on 101.