The pure Python code in the last example is more verbose than it needs to be.

    groups = {}
    for row in filtered:
        key = (row['species'], row['island'])
        if key not in groups:
            groups[key] = []
        groups[key].append(row['body_mass_g'])

can be rewritten as:

    groups = collections.defaultdict(list)
    for row in filtered:
        groups[(row['species'], row['island'])].append(row['body_mass_g'])

and

    variance = sum((x - mean) ** 2 for x in values) / (n - 1)
    std_dev = math.sqrt(variance)

as:

    std_dev = statistics.stddev(values)