As you can tell from their scientific names, they belong to different species. what taxonomic ranks do the medium ground finch (geospiza fortis) and the cactus finch (geospiza scandens) share?

Geospiza is the study of bird species.

Medium ground finch (Geospiza fortis) and cactus finch (Geospzia scandens) they do share some taxonomic ranks

(i)                  They both have a genus of birds in the family of Thraupidaes.

(ii)                They are endemic to the Galapagos island.

(iii)               They are termed as Darwin’s finches.

(iv)              They belong to tanager family.

(v)                They contain hybrid species which nicknamed “BigBird” which is endemic to Daphne major.

(vi)              They both feed on primarily seeds. But can also feed on insects, buds, and young leaves.

(vii)             They are under parasitism of philornis downsi, as well as avian pox virus.

There are other taxonomic ranks which are not similar to each other for example:

(i)                  Geospiza fortis their natural habitat is subtropical or tropical dry shrubland and subtropical or tropical dry forest while Gespizia scadens their natural habitat is Arid lowland zones on each island.

(ii)                In Geospizia fortis their male plumage is solid black with white tips undertail and female plumage is brown and streaky.  Geospizia scaden which their male plumage is totally black while the female is brown and streaky.

The medium ground finch (Geospiza fortis) and the cactus finch (Geospiza

scandens) share all taxonomic ranks except Species.

The taxonomic ranks which these organisms share include the following:

They are different species which is why they have different last names but

possess similar characteristics.

Read more about Taxonomy here link