The difference, by the numbers
They have no ingredient in common: the Jungle Bird has dark rum, Campari, pineapple juice, lime juice and Demerara syrup, the Monkey Gland gin, orange juice, absinthe and grenadine. In parts, a Jungle Bird is 3 rum, 1½ Campari, 3 pineapple, 1 lime and 1 syrup; a Monkey Gland is 3 gin, 3 orange, 1 absinthe and 1 grenadine. By volume, the Jungle Bird is 32% mixer and juice, 32% spirit, 16% fortified wine and bitter, 10% syrup and sugar and 10% citrus; the Monkey Gland is 38% mixer and juice, 37% spirit, 13% syrup and sugar and 12% fortified wine and bitter. Different bottles, similar balance: that makes them close cousins on the ratio map.
The Monkey Gland is the stronger drink in the glass (14.0% ABV against 10.9%) and holds more alcohol (1.52 standard drinks against 1.32). Calories are close (about 216 and 223), but the Jungle Bird has more sugar (19.7 g against 13.0 g). Both are shaken; the Jungle Bird is served in a rocks glass, the Monkey Gland in a coupe.