After some thought, you decide to go with the common house sparrow. Perhaps it's because how common the bird is, or how generic a bird it comes across as, or maybe you even respect its hardiness and resilience to survive across the globe. Maybe it's the first thing that comes to mind when you think of birds. Whatever reason you use to justify it, you pick up the sparrow "egg" and pop it into your mouth.
Your heart is beating fast. You're not sure what to expect out of this, but the thought that you're finally going to become a bird is giving you a thrill. You look at your hands as your breathing grows labored. No, nothing happening yet, although you feel a pleasant tingling starting to spread across your body. You quickly walk over to the nearest window and open it, leaning on the windowsill and taking a few deep breaths.
The tingling turns into an itching that spreads across your arms. You hold them in front of you to finally see the first signs of your transformation: little pinfeathers pushing their way out of your skin, like so many tiny needles. It doesn't hurt, just feels mildly uncomfortable and definitely strange. As you watch, the ones on the upper surface unfurl into small, soft brown feathers, while the ones on the lower part of your arm grow longer and longer. You realize that these are your primaries, the feathers that will allow you to fly as a bird.
Speaking of flying, other changes are starting to happen to prepare you for the air. You wince, arching your back as your chest pushes outwards. You can feel your breasts deflating and flattening out. A careful inspection under your clothes tells you that even your nipples are gone. Birds don't have those. Instead, you gasp as the muscles of your arms and chest begin to swell, becoming powerful flight muscles. Your sternum continues to push outwards to form an increasingly distinct keel that will anchor your developing muscles.
As your upper body bulks up, your lower body loses its previous importance. There is an audible crack as your hips cave inwards, becoming a narrower pelvis better suited for laying eggs than birthing babies. Your shoes are getting tight, and you kick them off, finding yourself now standing on tiptoes as your metatarsals lengthen. You're being forced into a horizontal stance by your shifting proportions, and you grip the windowsill tightly as your transformation progresses.
You can feel the clothes you were wearing sliding off your narrow hips. Were you in jeans? A skirt? Pajamas? Underwear? It doesn't matter, it doesn't fit anymore. The only thing on you now is what you were wearing for a top, and it fits oddly on your altered proportions. The feathers are growing down your back and torso, forming a line of brown plumage that spreads over your skin. At the base of your spine, you feel brief but intense pressure as your coccyx pushes out little by little, forming the little nub of a sparrow's tail. Long feathers, like the ones now gracing your arms, rapidly sprout on it as well, and you feel a surge of exhilaration as you realize you can move it. A less pleasant sensation is that of your genitals and anus merging together, forming a cloaca, but that passes quickly at least.
Now more bird than woman, you feel the tingling changes move into your face. Your lips and jaws tighten, then start slowly pushing out and hardening. You realize that you can't move your lips anymore as they solidify and turn into hard, light brown keratin, while your nose soon joins them, pushing out and melting into your growing, distinct beak. The feathers spread over your face as well, replacing the hair on your head which is progressively absorbed back into your scalp. Your eyes, whether they were brown or blue, green or grey, turn into the black, bright eyes of a sparrow.
You flap your arms experimentally as their plumage becomes more complex, differentiating into primary and secondary flight feathers. Your second and third fingers stretch out while your thumb shrinks and your other fingers shrivel away and vanish entirely. You can no longer grip the windowsill, but realize you can move your long arms like a bird now. You can feel your legs itch as scales cover them. One of the toes on each of your feet disappears, but the others grow, spreading out and sprouting long, curved claws. You flex your legs, seeing how you can now grip branches tightly.
By now you're almost entirely a roughly human-sized sparrow, but that changes quickly too. You feel an intense pressure on your entire body, as if someone is compressing you from all sides. Everything in the room seems to move away from you, and you feel a rush of nausea as your viewpoint shifts. That's when you realize that you just shrunk! You don't have time to adjust to it before you shrink again, and again. Your top is like a tent on you, and you shake it off with a bit of fluttering.
Everything seems so big to you now! But you're not done shrinking yet, not until several more shrinking spurts hit you and you've dwindled to the size of a house sparrow.
You would cry for joy, but you don't have the tear ducts for it, so instead you hop and flutter with unconstrained excitement. It worked! You're a sparrow now! You unfold your wings and inspect them, little marvels of engineering, spreading your primaries and looking at every feather. You turn around, marveling at how quick and agile you've become, and try hopping like a sparrow, chirping happily when it works.
Everything looks so different. Your bird eyes are far sharper than a human's, and it feels to you as if you're seeing your room for the first time. You hop in one direction, then another, trying to see as much as you can.
But that's not the main attraction, is it? You spread your wings. Do you feel confident that you can use those? Or apprehensive now that you finally have the chance? You take a deep breath and, aiming firmly for the windowsill, flap your wings as hard as you can.
Your tiny feathered body rockets through the air at terrifying speed. You remember to deploy your legs at the very last second, gripping onto the edge of the windowsill and hanging on for dear life.
You did it. You flew for the first time in your life.
Heart beating out of control, you hop to the edge of the windowsill, taking in your options.