As the 20th century progressed, the population rose higher, as did the buildings within metropolitan areas. In 1922 a patent was filed by Washington resident Emma Read to help alleviate the problem with urban congestion, providing a method to keep babies "healthy" by placing them in cages outside of windows.
As these photos from the 1930's show, the popularity of the devices grew over the next few decades, especially in the United Kingdom.
From the August, 1953 issue of Popular Mechanics:
"Enclosed in a wire cage suspended from an apartment window, English children play in the sunlight and fresh air while their mothers are busy with housework. The cage, made of wire netting is strongly braced and is guarded on the apartment side by a cloth net which prevents children from crawling back into the room. Loaned by an infant welfare center to families with no gardens, the portable balcony is apparently popular with children and mothers. The demand exceeds the supply."