A couple of weeks ago, after heavy rain, a bright yellow gooey patch appeared on one of our older towers: Gentle (pigeon tower n.4).
This patch started out looking elastic and glutinous, and seemed to dry out as the days passed, becoming white and then darkening in colour until it turned, about a week later, dark purple/black and powdery.
We identified this as Fuligo Septica, commonly known as Dog Vomit slime mould, or even Scrambled Egg slime mould. It grows on bark mulch, rotten wood and plant debris, so the towers are a the perfect habitat.
They are able to roam in an ameboid-like fashion, and they do so to search for nutrients to consume. They are predators that eat microorganisms like fungi and bacteria, playing a crucial part in the transformation of the tower into compost by decomposing and recycling nutrients. It eventually degrades, generating spores that are dispersed by the wind and beetles, these can remain viable for years, waiting for the right conditions to suddenly appear again.
In Finland this slime mould was believed to be used by witches to spoil their neighbors' milk, perhaps this explains the Dutch name for F. Septica, which translates to "Witches Butter".