Creation
Like all spirits, bosprits form around a Focus - a physical object that becomes the core of their being. A bosprit's Focus is always a bird skull.
As things live, grow and die, they leave behind spiritual energy. This energy is the equivalent of leaf litter, and it builds up over time. Parts of it will pool around the focal object. Eventually, this pool of spiritual energy reaches critical mass and forms a nascent spirit. This could be considered a 'baby' bosprit - it doesn't have a solid form yet, but it is it's own distinct spirit, with a Focus and whatever other physical objects (plants, rocks, fungi, bones) that it incorporated. Over time, this baby bosprit will settle into a solid form, although bits of its spirit may still be evident. This is considered a mature bosprit. A young but still mature bosprit may have animal levels of intelligence, but as they grow and consume more spiritual energy, they may become more intelligent and powerful. Whether that intelligence is more 'human' is up for debate.
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The time that this process takes is highly dependent on the amount of spiritual detritus available. Completely barren landscapes cannot support spirit growth, while highly fertile environments like rainforests may support many spirits of all kinds. It's also worth noting that 'barrenness' is determined by the presence of all kinds of life, as well as diversity. For example a monoculture plantation will be more barren than a thriving wild grassland. Similarly, a pile of compost that doesn't support much vertebrate life may be thriving with insects and bacteria.
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Bosprits generally take about 15 years to be 'born' and then another 5 years or so to grow from their 'baby' form to a mature form. A mature bosprit may live a long time in that state - 50 to 100 years is very common. Spirits also have a life cycle however; mature bosprits may continue to grow in size, intelligence and magic. On average, the maximum size a bosprit may reach is 30cm.
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If the bosprit makes it to old age without fading away, and continues to grow in spiritual power, it may ascend to a greater spiritual form. Bospirits are ancient bosprits, and are far, far rarer. They're larger than your average bosprit, and may have an extra pair of feet, wings, tail or a long neck. They almost certainly have 'human-level' intelligence.
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More on ascended spirits can be found here: