Gold is a lustrous heavy and precious metal, mostly prized for its color and weight. It’s been used as currency and as a precocious desired thing for as far back as history books will ever go. It’s still used as a standard “best metal” in the Olympic Games and many, many other things. Jewelry is its main use, often alloyed with silver or copper to help its strength and help prevent corrosion. It’s also used in electronics and computing.
Lions are large carnivores that used to live in most of the Eurasian continent. They still form the crests and symbology of many countries. Especially that of England which likely hasn’t had native lions for at least 2 centuries. They now mostly reside in Africa and some central Asian countries. They often have gold coats (also yellow, grey, light brown) and this is why they are tied to gold.
Mercury is a toxic metal that is liquid when at room temperature. It’s often very hard to handle and use in a practical way. Though it used to be used in the dying of top hats. In fact the Mercury would be found to be the culprit of many poisoning’s that led to madness, liver damage and of course death. It inspired the Mad Hatter Alice in wonderland and thus helped coined the phrase “Mad as a Hatter.” Its still used in thermometers, weather measuring instruments and thermostats, compasses, some dental applications (in very small amounts) and in some types of lighting.
Messenger pigeons are technically a specific bread type of domestic pigeon that has its home finding abilities selectively bred. Despite it not being a actual species it made sense that Messenger pigeons matched with Mercury than the Domesticated pigeons. The main reason being that most domestic pigeons would not travel as far, and mercury would when used as for compasses.