Chaelis

The Chaelis were an avian gestalt consciousness that resembled blue hummingbirds.

Biology
Although they had poisonous claws (the poison a byproduct of their biological research), the Chaelis had evolved along another creature, "Abraham," that acted as their host, the Chaelis covering him and gripping him with their claws and beaks. The Seventh Doctor theorised that this is how the flock was able to manipulate tools and build spaceships, using some form of short-range telepathy or thought-control. He believed the birds were probably relatively mindless individually, but as a flock their group intelligence could influence Abraham. (AUDIO: A Thousand Tiny Wings)

Personality
The Chaelis referred to each other as "Brothers." When pressed by the Doctor why they didn't conduct research on their own world, the Chaelis responded that it was too dangerous, having no qualms about affecting life on another planet with their biological testing: "We do what works. Morality is a luxury." They were smug in telling him they had developed a cure for their microbes.

The Chaelis believed warfare was a foolish past-time, wasteful of energy and resources, but intended to sell their weapons to other races, gaining control of the galaxy by becoming the most powerful economic entity. (AUDIO: A Thousand Tiny Wings)

Technology
utilising creatures like Abraham as mules, the Chaelis had built spaceships to travel between worlds.

They were adept at biological engineering, creating an artificial microbe that could infect people. As a precaution, the microbes all contained a genetically engineered weakness, a biological trapdoor that could be exploited by a particular toxin as the cure. (AUDIO: A Thousand Tiny Wings)

History
In Kenya, 1953, the Doctor and Elizabeth Klein encountered the Chaelis conducting research for biological warfare testing in their dome. Not only was Earth "convenient" as the atmosphere matched theirs and they could eat the insects while Abraham ate the vegetation, but of all the planets surveyed, Earth had the highest number of microbial organisms and the fastest rate of evolution, making the planet "unique." They also considered the dominant species humanity to be another advantage, forming "perfect test subjects" that could become infected by the artificial microbes.

They planned to sell the microbes to other species engaged in warfare, claiming they would pay highly for microbes that could infect their enemies, and when the other races had wiped each other out or bankrupted themselves, the Chaelis would become the most powerful economic entity in the galaxy and would control it. The Doctor defeated them by speaking directly to Abraham, urging him to fight back as he made the Chaelis repeat the words Klein spoke to him. Abraham reversed the telepathic link, killing some of the Chaelis and scattering the others, leaving them unable to form a group intelligence. Acting only on instinct, the individual Chaelis attacked Klein and the Doctor, who fended them off, killing them. (AUDIO: A Thousand Tiny Wings)

Behind the scenes
The spelling for "Chaelis" comes from the script for A Thousand Tiny Wings.