The Plague struck Scotland in the middle of the 14th century.

"In 1350, there was a great pestilence and mortality of men in the kingdom of Scotland, and this pestilence also raged for many years before and after in various parts of the world. So great a plague has never been heard of from the beginning of the world to the present day, or been recorded in books. For this plague vented its spite so thoroughly that fully a third of the human race was killed. At God's command, moreover, the damage was done by an extraordinary and novel form of death. Those who fell sick of a kind of gross swelling of the flesh lasted for barely two days. This sickness befell people everywhere, but especially the middling and lower classes, rarely the great. It generated such horror that children did not dare to visit their dying parents, nor parents their children, but fled for fear of contagion as if from leprosy or a serpent." John of Fordun (d.1384), Scotichronicon

An ancient nursery rhyme called 'Ring -a-Ring of Roses' goes :-

Ring o' ring of roses
A pocket full of posies
Atishoo! Atishoo!
We all fall down."

The popular children's nursery rhyme, 'Ring-a-Ring of Roses,' describes the rapid progression of sickness as it quickly led to death. 'Ring-a-Ring of Roses' refers to the raised red/blackish spots that eventually turn to skin boils and blisters associated with bubonic plague, while 'a pocket full of posies' recalls the vain attempts to purify the air and ward off "evil fumes" by carrying packets of sweet smelling herbs. 'Atishoo atishoo' describes the later symptoms of the pneumonic plague, causing coughing and sneezing, while 'we all fall down' speaks graphically for itself as the final death of the victim! See, http://www.borderdisc.com/mag/blackdeath.htm

The pneumonic plague would have likely threatened all of the Caldwell who made their living either in a village or with other families clustered in a hamlet. Possibly they abandoned the Lord of Little Caldwell manor and Mure of Caldwell Manor and sought to protect themselves from the pneumonic plague by relocating to more sparsely occupied areas of Ayrshire and Renfrewshire, the very locations identified herein by David of Manitoba and Tom of Australia.

I conjecture that this may be the reason each of the Caldwell farms displayed on Toim's maps is isolated from the other Caldwell farms.