Air carrier pilot reported refusing an aircraft after finding bird remains inside of engine.

Narrative

On Day 0; during our preflight inspection of Aircraft X; we identified significant evidence of bird ingestion into the #1 engine from the inbound flight. We observed multiple pieces of bird remains--including guts and feathers--on and around the #1 LP (Low Pressure) fan blades; inside the LP bypass area; and splatter/staining consistent with ingestion. The spinner also showed signs of bird impact. Further inspection revealed that the #1 LP stator vanes had visible feathers and bird remains on the leading edges.We contacted maintenance; who conducted a visual inspection. The mechanic stated he ran his hand along the blades and stator vanes" and reported no damage. The discrepancy was signed off referencing the finding as "evidence of a possible bird ingestion." We disagreed with this characterization--there was clear evidence that at least one bird of unknown size had entered the #1 engine. No confirmation was provided regarding whether the bird had passed through the engine core.Following the maintenance action; we performed an additional preflight and found that bird remains were still present. This included a partial bird wing in the bypass and additional solid remains on the #1 LP stator vane leading edges. Given the remaining bird-related FOD and the unresolved possibility of internal engine damage; we did not consider the aircraft airworthy until a borescope inspection had been performed and verified no engine damage. In the interest of safety; we refused the aircraft until such inspections could be completed."