Answer:
39
Step-by-step explanation:
Somatic or body cells are diploid and are denoted by 2n number of chromosomes. During meiosis, specifically after meiosis I in cell division, the chromosome number is reduced to half the number of that of the somatic cell, and is referred to as haploid, which is denoted by n number of chromosomes.
Therefore, if the somatic cell of the bird has 78 chromosomes, the sex cell (unfertilized egg or sperm) would contain half of the number of chromosomes of the somatic cell = 39 chromosomes.