Final answer:
The author includes the numbers of birds sighted in recent 'big years' to highlight trends in avian populations, illustrate the state of conservation efforts, and underline the urgency of protecting species and their natural habitats.
Step-by-step explanation:
The inclusion of the numbers of birds sighted in recent 'big years' by authors serves multiple purposes in the context of avian species conservation and biodiversity studies. These numbers provide a snapshot of changes in bird populations and can be used to illustrate trends in extinction rates, discovery of new species, and the impacts of habitat loss. For instance, the Pimm et al. (2006) study emphasizes the crucial gap in extinction accounting due to the fact that scientific descriptions of species only began properly in the 1700s. This signifies that many species potentially went extinct before they were ever described, thus their loss was not recorded.
Moreover, these numbers reflect the efforts of the conservation community in tracking and understanding the survival challenges of birds. Data showing that since 1950, nearly 300 new bird species were added literally depicts an ongoing process of discovery and taxonomy, while also highlighting the perilous state of many species: 48% of these are threatened or already extinct. This illustrates the dire situation for bird species and the importance of monitoring and protecting them.
Additionally, such counts underscore the importance of natural habitats and the consequences of their destruction. It is noted that there are bird species that may never be described if their habitats are eradicated before discovery. Hence, including bird sighting numbers in 'big years' can be central to accentuating the urgency of conservation work and the challenges faced in the documentation of Earth's biodiversity.