There were an estimated 3-5 billion passenger pigeons in the Americas before Europeans arrived.

There are currently about 1.5 billion cattle worldwide, and each one weighs a lot more than 3 pigeons.

Worldwide livestock biomass significantly exceeds worldwide wild animal biomass: https://xkcd.com/1338/

The current livestock population can only be supported by industrialised agriculture.

Livestock biomass is also artificially increased both by many generations of selective breeding and by a known effect of certain hormones and antibiotics.

The actual methane emissions are a lot more complex than just count or biomass, as demonstrated even by the headline of the link (you wouldn’t see 55%, let alone 80%, if it wasn’t): https://journals.plos.org/plosone/article?id=10.1371/journal...

Finally, this planet does not actively manage global warming. It goes though glacial and interglacial periods precisely because of the absence of active management. The natural causes of this lead to the pre-industrial cycles, and are why the average temperature of this planet is so much higher than the average temperature of the moon: http://www.wolframalpha.com/input/?i=average%20temperature%2...