This line, "straw bed," helps create the image of a bird's nest, and it also conveys a sense of poverty and the mother's inability to provide a luxurious life for her daughter.
In Gabriela Mistral's poem "Fear," a mother speaks about her daughter, voicing her fears about how some day her daughter will grow up and leave her.
First, by explaining how she doesn't want her daughter to turn into a "swallow," which is a type of bird, and then saying that her daughter may never return to their "straw bed," (which has been otherwise translated as "mattress," with the same implications) the speaker of the poem likens herself to a mama bird, her daughter to a baby bird, and their home to a nest. By comparing herself and her daughter to birds, the speaker expresses the universal and basic fear of separating from her daughter. The comparison also helps us imagine the daughter as a creature who will, naturally, want to fly free from her mother's protection some day.
Second, the "straw bed" or "mattress" line also helps express the mother's poverty. Anyone sleeping on a straw bed or just a mattress is, probably, poor. When you imagine sleeping on a literal straw bed, you get an image of sleeping in a barn, or a rundown shack, or a hut from a book by Laura Ingalls Wilder. This expression of poverty is important in the speaker's anxieties: she doesn't want her daughter to fly away (to leave her to make a life of her own) and yet the speaker isn't able to provide the kind of luxurious life that would tempt a daughter to not even look for an easier, more comfortable life out on her own.