Humans do not have a monopoly on the magic of Atheria, no matter how much they sometimes wish they did. Fortunately for them, however, they do tend to have more of it than most creatures do.
A creature’s ability to use the magic of Atheria is dependent on two major factors – Intelligence and Size.
Intelligent creatures can almost always tap into the full powers of their birthrights, and often in more sophisticated ways than unintelligent creatures. The standard isn’t even all that high: only the most severely damaged humans are unable to use their birthrights effectively. Intelligent beings are, however, subject to the Ban of the Fey – limiting the powers of their Birthrights, or at least those granting spell-like and spellcasting abilities, to effects of level three or less. Whether unfairly or not, nonsentient animals are not so limited – although they will rarely have more than one or two powerful tricks.
Intelligent beings with who train and study their birthrights may enhance their birthrights up to the +1ECL level, but this appears to be the limit.
Interestingly, if you grant intelligence to an unintelligent creature, it’s Birthright will change – losing the higher-order effects (if any) in favor of increased flexibility.
Similarly, size matters – at least in unintelligent creatures. Small insects may have some tiny enhancement – a nastier bite or toxin, an improvement on their camouflage, or some such (1-2 CP at the most) – but you will not find a plague of birthright-wielding ants or other “fine” creatures depopulating the wilderness. Swarm creatures are an occasional exception: the colony as a whole may be capable of protecting itself with some minor effect (6 CP at most). Rats, mice, songbirds, and other diminutive creatures may have some minor auxiliary tricks (3-4 CP), but for the most part their nature and habits are familiar throughout Atheria.
The larger rabbits, weasels, and other “tiny” creatures may have a limited-use special “trick” or two (worth up to 12 CP) or – most commonly – they may simply enhance their natural talents with a bit of magic. The local rabbits may react, run, or dodge unnaturally quickly, use small illusions to help camouflage themselves and conceal their hiding places, bend time or space for an instant to evade an attacker, simply need less to eat and drink, or sense the approach of predators, but the overall strategy is usually pretty recognizable. Predators are less predictable: for them, a few one-shot tricks are often in order.
Small creatures – such as common dogs, large birds, and human infants (while they’re still too young for their intelligence to override the mass-based limit) may have substantial birthrights – wielding powers worth up to 24 CP. Perhaps unfortunately, in human children this often means unreliability (corrupted or specialized) rather than less power – making childhood with some of the more dangerous birthrights even more deadly.
Medium-sized creatures may have a full +0 ECL Birthright. Each additional size category raises this limit by +16 CP.
Unintelligent plants and fungi are a different matter: In general, such organisms only show passive adaptions: they may be larger, tougher, toxic (fairly common), have spikes or thorns, or able to thrive in otherwise unfriendly environments (extremely common), but plants and fungi which actively attack are relatively unusual. At least as importantly, they don’t seem to channel magic as effectively as animals do, considerably reducing the level of the abilities they display.
Oddly – and perhaps a betrayal of the hand of the Fey – more impressive, attractive, and interesting animals tend to have more unique and intriguing Birthrights. There are quite a few varieties of mustelids for example – yet most weasels, minks, ermines, and ferrets in a domain will share a single birthright, while otters and sea otters usually differ. Why? Apparently someone powerful among the fey likes otters.
Despite this, Birthrights are difficult to change. It takes a great deal of tinkering, usually over many generations, to modify a species Birthright.
All of this makes for some unique dynamics when farming. The same species may prove radically different depending on which domain it’s born in. On the other hand, it contributes enormously to trade: worried about spirits? Get a spirit-sighted guard dog born in Chelm. Want to train a dog to do fancy tricks? Get one from Atheria. Want one that can seriously protect your children? Get one from the Warding domain. Windsteed? HuSung. Stormhawk? Parack.
Just as importantly, since Birthrights depend on the place of birth, there’s no use trying to breed your own specimens at home. Stormcloaks – which deflect the ravages of weather, allowing the wearer to easily withstand gales, remain dry in downpours, and comfortable in blizzards, can only be woven from the wool of sheep born in Parack, and that’s it.