Unknown to the National Wildlife Federation, perhaps, but an endangered species nonetheless, is the breakfast eater. While many still exist, polls show that their numbers shrink each year.

Scientists give various reasons for the declining numbers. Late nights make late risers who, understandably, are late for work. They disregard the need for a morning meal.

Late dinners postpone the gnawing feeling that would otherwise force one to indulge in something edible before noon. Late diners disdain breakfast.

And as family members become distracted by early morning activities (running, flex-time commuting or meetings at school), breakfasts shrink and become less important.

None of these, however, reflects badly on the actual food, which is very often delicious and certainly easy to prepare. Often people look forward to vacation just because they can indulge in the ham, grits, potatoes, hash, sausage, eggs, waffles, pancakes, kuchen and croissants that breakfast connotes.

For those who, as a general rule, cannot or will not face a substantial meal before noon but enjoy the thought of a hearty morning meal, and for their counterparts who fantasize about that food, consider this schizophrenic menu: It can be dinner or breakfast.

As always, every kitchen is presumed to be equipped with flour, sugar, salt, pepper and butter or oil.

EXPRESS LANE LIST: Eggs, canadian bacon (or small, boneless ham), english muffins, vinegar, mangoes, lime, honey. POACHED EGGS AND CANADIAN BACON (4 servings) 4 1/4-inch-thick slices canadian bacon About 1 tablespoon butter or oil 4 english muffins 1 tablespoon vinegar 4 to 8 eggs (1 to 2 per serving)

Fry canadian bacon slices in a little butter or oil to heat through. Place on a platter and keep warm in the oven. Halve muffins and toast. Heat enough water to cover eggs, simmer in a large skillet with 1 tablespoon vinegar. Crack eggs individually into a flat saucer and slide them into hot water. Cook until done to taste and remove with slotted spoon. Drain eggs briefly on absorbent towels, then serve atop bacon slices on heated plates. Canadian bacon can be placed on top of muffin halves, or the muffins may be buttered and served with honey on the side. SAUTE'ED MANGOES (4 servings) 3 tablespoons butter 2 ripe mangoes, peeled and sliced 2 limes 3 tablespoons honey

Heat butter in a large skillet. Add mango slices, juice from limes and honey. Heat through and serve with poached eggs.

If ripe mangoes are not available, substitute other ripe fruit such as banana halves, papaya, plums, pears or apples. (Cook to desired doneness.) You may prefer bananas and plums sprinkled with cinnamon, pears with ginger and apples with cinnamon and nutmeg. Or heat applesauce and serve it sprinkled with cinnamon.