What you see in the picture below is a nest, painstakingly constructed by a bird name "Hornero" (Furnarius rufus). The Spanish name could loosely be translated as "Oven Maker".

The nests are made over several days using mud and pieces of straw and its constructor manages, nearly always, to place the opening looking north. You must remember that in the southern hemisphere, the cold and prevailing winds are from the south, therefore the north side is more protected from the elements. 

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Obviously, such a nest created havoc with the insulation between lines and to earth, giving place for crosstalk or signal losses.

Before our ecological times, it was common practice by the linesmen to destroy the nest to avoid the problems mentioned above. Another solution, adopted later, was to install between insulators a wire structure which prevented the "Hornero" from constructing the nest on the crossarms. 

This bird is common in Uruguay and parts of Argentina, Brazil, Bolivia and Paraguay. It stands no more than 7" in height.

I recall that many years ago a nest was sent to the Head Office in England of a telegraph company operating in Uruguay, as an example, I presume, of the unexpected difficulties you could encounter when running a telegraph line. Nowadays, you can see a lot of these nests on abandoned telegraph lines which have now become a safe perch for this industrious bird.