/**
     * Return the first Sighting object from a
     * particular animal name and spotter ID.
     * @param animal The type of animal.
     * @param spotterID The ID of the spotter.
     * @return An Optional of type Sighting in the sightings collection for
     * that combination.
     */
    public Optional<Sighting> getFirstSighting(String animal, int spotterID)
    {
        return sightings.stream()
            .filter(sighting -> animal.equals(sighting.getAnimal()))
            .filter(sighting -> spotterID == sighting.getSpotter())
            .findFirst();
    }