Chickens count from the left, just like us!

Rugani et. al. show (Brugger's summary) that

...3-day old chicks associate small numerosities with the left side, and large ones with the right side, of a given space. The results show that newborn chicks can understand both relative and absolute quantities, and also suggest that the brain may be prewired in how it relates numbers to space. The work casts doubt on the importance of language and symbolic thought for the ability to represent discrete quantities larger than 3 and to develop a sense of numerical order and counting routines. Field studies of avian behavior have previously documented this ability in adult birds.

Abstract:

Humans represent numbers along a mental number line (MNL), where smaller values are located on the left and larger on the right. The origin of the MNL and its connections with cultural experience are unclear: Pre-verbal infants and nonhuman species master a variety of numerical abilities, supporting the existence of evolutionary ancient precursor systems. In our experiments, 3-day-old domestic chicks, once familiarized with a target number (5), spontaneously associated a smaller number (2) with the left space and a larger number (8) with the right space. The same number (8), though, was associated with the left space when the target number was 20. Similarly to humans, chicks associate smaller numbers with the left space and larger numbers with the right space.

More from Brugger's summary:

A more specific insight from Rugani et al.'s study is that a chick's sense of numerical order is tightly coupled with its sense of space: “More than” is equivalent to “to the right of.” This leads to a left-to-right directionality in the mapping of numbers to space—a finding that puts several previous proposals for the origin of mental number lines into perspective. One reason why researchers have assumed that this kind of numerical mapping is an invention of the human mind is its cultural modification. In cultures with a left-to-right reading and writing direction, the number line expands from left to right, but cultures with an opposite directional handling of script align numbers from right to left. Obviously, reading/writing direction cannot be the ultimate cause of directionality, nor can finger-counting habits. Presumably, the predominant role of the right hemisphere for numerical ordering biases initial attention to the left side of both physical and number space. Together with a preference for increasing over decreasing order—already apparent in 4-month-old human infants—the biological default of a number line would point from left to right.