**Answer**

There are approximately 10,000 known species of birds in the world. However, it is important to note that new species are constantly being discovered, and the total number of bird species may change over time.

**Explanation**

*Birds are a diverse group of animals belonging to the class Aves. They are characterized by feathers, beaks, and the ability to fly. Birds can be found in almost every habitat on Earth, from the Arctic tundra to tropical rainforests.*

1. **Estimating the Number of Bird Species**:
- The estimation of the number of bird species on Earth is not an easy task due to several reasons.
- Birds are highly mobile and can migrate across continents, making it difficult to accurately count and classify them.
- Additionally, there are remote and inaccessible regions where bird species may exist but have not yet been discovered or studied.
- Despite these challenges, researchers and ornithologists have been able to identify and describe a significant number of bird species.

2. **Current Number of Known Bird Species**:
- As of now, there are approximately 10,000 known species of birds.
- These species have been identified and described through extensive scientific research and field observations.
- The number of known bird species can vary slightly depending on different taxonomic classifications and revisions.

3. **New Bird Species Discoveries**:
- Discovering new bird species is an ongoing process, and new species are still being identified.
- Scientists and ornithologists regularly conduct fieldwork in various regions, including remote areas, to document and describe new bird species.
- Advancements in technology, such as DNA analysis and molecular techniques, have also facilitated the identification of new bird species.

4. **Importance of Documenting Bird Species**:
- Documenting and understanding the diversity of bird species is crucial for several reasons.
- Birds play vital roles in ecosystems as pollinators, seed dispersers, and predators of pests.
- Studying bird species provides insights into biodiversity, evolution, and ecological dynamics.
- Conservation efforts rely on accurate information about bird species to protect and manage their habitats effectively.

In conclusion, while the exact number of bird species on Earth may not be known with absolute certainty, the current estimate stands at approximately 10,000 known species. Ongoing research and discoveries will likely continue to add to this number in the future.