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The Gemara describes the process of Melikah, the slaughtering of the bird-offering by hand. The Beraisa concludes that this is the most difficult Avodah to perform in the Beis ha'Mikdash. The Gemara asks that there are other Avodos which are more difficult, such as the Avodos of Kemitzah and Chafinah (see Yoma 47b and 49b).
The Gemara answers that the Beraisa does not mean that Melikah is the most difficult Avodah, but rather that it is among the most difficult Avodos in the Beis ha'Mikdash (see RASHI, DH Avodah Kashah).
Midrash Rabba Vayikra Rabba 3:5 focused on bird sacrifices, where both the meal offering and the bird offering are sacrifices typically given by the poor who could not afford the more expensive animal sacrifices.