from typing import List, Optional
import re

def validate_bird_name(name: str) -> bool:
    """
    Validate the bird name.

    Args:
        name (str): The name of the bird.

    Returns:
        bool: True if the name is valid, False otherwise.
    """
    pattern = r'^[a-zA-Z\s]+$'
    return bool(re.match(pattern, name))

def validate_bird_species(species: str) -> bool:
    """
    Validate the bird species.

    Args:
        species (str): The species of the bird.

    Returns:
        bool: True if the species is valid, False otherwise.
    """
    pattern = r'^[a-zA-Z\s]+$'
    return bool(re.match(pattern, species))

def validate_bird_age(age: Optional[int]) -> bool:
    """
    Validate the bird age.

    Args:
        age (Optional[int]): The age of the bird.

    Returns:
        bool: True if the age is valid (non-negative integer or None), False otherwise.
    """
    if age is None:
        return True
    return isinstance(age, int) and age >= 0

def get_bird_description(name: str, species: str, age: Optional[int]) -> str:
    """
    Generate a description for a bird based on its name, species, and age.

    Args:
        name (str): The name of the bird.
        species (str): The species of the bird.
        age (Optional[int]): The age of the bird.

    Returns:
        str: The generated description for the bird.
    """
    description = f"{name} is a {species}"
    if age is not None:
        description += f" that is {age} years old"
    return description

def filter_birds_by_species(birds: List[dict], species: str) -> List[dict]:
    """
    Filter a list of birds by species.

    Args:
        birds (List[dict]): A list of bird dictionaries.
        species (str): The species to filter by.

    Returns:
        List[dict]: A list of bird dictionaries matching the specified species.
    """
    return [bird for bird in birds if bird['species'] == species]