This picture is a bufflehead. Like many ducks it nests in abandoned cavities, but in this case it is almost always a nest excavated by a northern flicker. Presumably the reason for the symbiotic relationship is caused by the similarity in size requirements between the two birds.
When it comes to nesting cavities there are several relevant measurements. The size of the inside of the cavity, the size of the opening and how high the cavity is off of the ground all come into play. The most important of these measurements is the size of the opening. It has to be big enough for the owner to fit through.
But you don’t want the opening to be any bigger. Predators tend to be larger than prey. Having a spacious opening means leaving the welcome mat out for whatever wants to turn you into a meal. There are, of course, some predators that can access a smaller hole, such as snakes, but hawks, owls, felines and canines will not pass through that small opening.
One of the key factors of security is controlling access. This is not just a rule in the physical world, it is also a spiritual problem. Have you noticed how many times in the Bible God’s people were admonished to remove something? Idols, those who practice witchcraft, foreigners and their gods all have to go in order to stop their spiritual influence. In a world that prides itself on inclusiveness being spiritually discerning is difficult.
The only way it really works is to take God’s Word seriously and to stay away from those things that God makes clear is bad or unhealthy. This turns walking with God into a narrow path, and narrow ideas conflict with the open minded age we live in. But like the door into the nesting cavity, opening up the passage way will allow harmful things in.
Isaiah 14:6 (HCSB) “Therefore, say to the house of Israel: This is what the Lord God says: Repent and turn away from your idols; turn your faces away from all your detestable things.”