While he did include Sanchez, he felt there weren't many catching prospects who had outstanding seasons, so his inclusion was based on his age. As a 20-year-old he got his first taste of Double-A, hitting .250/.364/.380 in 110 plate appearances. He spent the majority of the season in High-A and hit .254/.313/.420 with 13 home runs, but his defense has been called into question by the organizations and scouts alike.
While Greg Bird is far off from the majors, he hit .288/.428/.511 with 20 home runs and an 18.7% walk rate as a 20-year-old in Low-A. While Sanchez is very young for the level he reached, Bird is still young for his level as well. He not only had one of the best seasons in the league, but he had more walks than anyone in baseball other than Joey Votto and Shin-Soo Choo.
Jonathan Mayo didn't include a single Yankee on his second team, which is fine, the Yankees organization doesn't have anyone in terms of frontline prospects. The issue is his reasoning:
I decided not to cave to Yankee fans bugging me about Greg Bird (who had a very nice year). Cron had a solid, if unspectacular, season in the Texas League.
So what he's basically saying is that he chose C.J. Cron, who he admits is "unspectacular" after hitting .274/.319/.428 with 14 home runs and a 4.1% walk rate in Double-A over Bird because he didn't want to give him the credit. Cron had a 107 wRC+ while Bird had a 170 wRC+.
On Twitter he defended his choice by claiming Bird didn't destroy the Sally League like Bryan Buxton, the No. 1 prospect in the game, did in High-A. The comparison is pretty unwarranted, but the RiverDogs play in a very pitcher-friendly ballpark, which is probably why he didn't "destroy" the league like Mayo apparently wanted to see. He hit a respectable 248/.383/.412 at home, but on the road he hit an incredible .328/.470/.608 with 15 home runs on the road. The thing is that Cron plays in a park that is even more pitcher-friendly, but he still only hit .264/.312/.407 on the road. To fault Bird for succeeding in an offense-devouring park, but then reward Cron for doing the same and playing worse doesn't make any sense.
It will take a few more seasons like 2013 to bring a non-first round first base prospect to the top of prospect lists, but his season should still be celebrated and he should still be given credit for what he did. People like to ignore Yankees prospects, which isn't unwarranted, considering their mediocrity, but evaluators can't claim to be impartial and not give Greg Bird the credit he deserves.