Ok, maybe it takes more than two birds to be a flock, but let’s go with this: BIRD + BIRD = FLOCK
Doable by hand!
Solutions are below the image
SPOILER SOLUTIONS!
| BIRD_first | BIRD_second | FLOCK_third |
|---|---|---|
| 5238 | 5238 | 10476 |
| 5239 | 5239 | 10478 |
| 5364 | 5364 | 10728 |
| 5382 | 5382 | 10764 |
| 5392 | 5392 | 10784 |
| 5436 | 5436 | 10872 |
| 5486 | 5486 | 10972 |
| 6354 | 6354 | 12708 |
| 6435 | 6435 | 12870 |
| 6485 | 6485 | 12970 |
| 6729 | 6729 | 13458 |
| 6792 | 6792 | 13584 |
| 6852 | 6852 | 13704 |
| 6927 | 6927 | 13854 |
| 7269 | 7269 | 14538 |
| 7293 | 7293 | 14586 |
| 7329 | 7329 | 14658 |
| 7692 | 7692 | 15384 |
| 7923 | 7923 | 15846 |
| 7932 | 7932 | 15864 |
| 8235 | 8235 | 16470 |
| 8352 | 8352 | 16704 |
| 8523 | 8523 | 17046 |
| 8532 | 8532 | 17064 |
| 8546 | 8546 | 17092 |
| 8645 | 8645 | 17290 |
| 8652 | 8652 | 17304 |
| 9235 | 9235 | 18470 |
| 9267 | 9267 | 18534 |
| 9273 | 9273 | 18546 |
| 9327 | 9327 | 18654 |
| 9352 | 9352 | 18704 |