The smallest bird among the options given is the hummingbird. Let's discuss each bird and why the hummingbird is the smallest.

1. Sparrow:
Sparrows are small passerine birds found all over the world. While they are small in size, they are not the smallest bird. They typically measure around 11-18 centimeters in length and weigh about 13-40 grams. Sparrows have a compact body with a rounded shape.

2. Calibre:
It seems like there might be a typo in the given option. The term "Calibre" does not refer to a specific bird. Therefore, we cannot consider it as an option for the smallest bird.

3. Ostrich:
The ostrich is the largest bird in the world, so it is definitely not the smallest. Ostriches are flightless birds native to Africa. They can grow up to 2.7 meters in height and weigh around 63 to 145 kilograms. Ostriches have long necks and legs, making them well-adapted for running rather than flying.

4. Hummingbird:
Hummingbirds are known for their small size and rapid wing movements. They are indeed the smallest birds in the world. The smallest hummingbird species, the bee hummingbird, measures about 5-6 centimeters in length and weighs around 1.6-2.6 grams. These tiny birds have vibrant feathers and can hover in mid-air, thanks to their unique wing structure. Hummingbirds feed on nectar and play a vital role in pollination.

In conclusion, the smallest bird among the given options is the hummingbird. Its petite size and remarkable flying abilities make it a fascinating creature to observe.

Humming bird