I have similar data for a slightly different time period (which of course makes sense) with accompanying data on the percentage of thoroughbreds born in each month on average from 1975-2015. It seems there may be a maturity advantage with earlier born horses as over 50% of the winners of the Kentucky Derby have been born in January and February, but only about 20% of thoroughbreds overall were born in those two months. Here is the data:
34% of Derby winners were born in January (7% of thoroughbreds born in that month)
24% of Derby winners were born in February (14% of thoroughbreds born in that month)
22% of Derby winners were born in March (27% of thoroughbreds born in that month)
12% of Derby winners were born in April (31% of thoroughbreds born in that month)
8% of Derby winners were born in May (17% of thoroughbreds born in that month)
*Note the remaining 3% or so of thoroughbreds in general are born after May
