Questions & Answers about В воде мы видим отражение обеих птиц.
Why is it обеих here instead of обоих?
The noun it modifies, птица (bird), is feminine. The word for "both" has two starting forms: оба for masculine/neuter and обе for feminine. In the Genitive case, the feminine обе becomes обеих, while the masculine/neuter оба would become обоих.
Why do we use the Genitive case for обеих птиц?
It translates the English word "of" to show possession or relationship: "the reflection OF both birds". In Russian, connecting two nouns this way puts the second noun phrase into the Genitive case.
Птиц ends in a consonant. Does that mean it is a masculine noun?
No, it is the Genitive plural form of the feminine noun птица (bird). When forming the Genitive plural for most feminine nouns that end in -а, you drop the -а entirely, which leaves it ending in a consonant.
Since отражение (reflection) is the direct object that we see, shouldn't it change its ending for the Accusative case?
It actually is in the Accusative case here! Because отражение is a neuter, inanimate noun, its Accusative form is identical to its Nominative (dictionary) form.