Some notes taken from Wedmore's book "Manual of Beekeeping"
Robbing Behaviour of Honey Bees is the robbing of honey from another colony of honey bees as opposed to that colony gathering nectar for itself.
What Wedmore says about robbing is as succinct a description as I can find, the following numbered paragraphs below are substantially as he wrote them, with modifications for modern language usage. (I think Wedmore's book is excellent. One of the great features is the numbering of paragraphs. R.P.)
1319. Bees are accustomed to gather their stores by laborious visits to many sources, each of which renders but a minute quantity. The finding of a source in the open, from which not only one, but many bees can load up on a single visit, causes great excitement among the bees. This is especially the case if the food offered is honey, not merely nectar or sugar syrup. Access to stores, and especially to honey, outside the home, excites their cupidity. If once a colony has experience of such "easy pickings" it may become accustomed to robbing its weaker and less aggressive neighbours.
1320. The tendency to rob varies with the race and strain. The beekeeper should not tolerate strains showing a strong predisposition to robbery. The vice is associated with the yellow races much more strongly than with the black or brown. Some of the lighter coloured bees are tractable in this respect, but in general the goldens or yellowest races are more predisposed than the leather coloured Italians. The American "Italian" is a mixed race and in America all "Italian" bees are suspect, not to an extent, of course, that hinders their free use on account of their many excellent qualities, but to an extent that necessitates precautions and watchfulness.
1321. Robbing is liable to be set up by any exposure of stores, and especially on cessation of a source of supply of nectar and by the temptation offered by poorly guarded stores or by leaky hives.
1322. Even if all precautions are taken, robbers will still try to rob, and it is well to keep a look out by observing the behaviour of the bees. When robbers are about, the bees will be on guard and the guards unusually active, challenging all who seek entrance. Occasional combats will be seen. The robber bee hovers around the entrance much as a wasp does, not seeking familiarity with the general appearance and surroundings as does a newly emerged bee, but seeking an opportunity to dart in past the guards. The robber bees are the older bees, not the newly emerged.
1323. In the height of the day the home bees leave the hive empty and return loaded, whereas the robber bee approaches the hive empty and leaves full. Now a flying bee with empty honey sac flies with legs extended behind, whereas a loaded bee bends the hind legs to bring them forward. The observation of this distinction is particularly useful in detecting slow robbing of a weak stock in cold weather.
1324. Some robbing and attempted robbing will occur in the best managed apiary, but if precautions are taken it should not develop into raiding and destruction. If the beekeeper has brood disease to contend with, a check on robbing is most important, as the diseased and disheartened and weakened stock is particularly liable to attack, thus leading to the disease being spread to healthy colonies.
1325. The danger of robbing is minimized by removing temptation. A cover cloth or carbolic cloth should be at hand as a temporary cover for hive bodies exposed during manipulations. When robbing is feared hives should be examined late in the day. Outer cases should be bee-tight and covers also. Robber bees are quite capable of handing out the goods to their friends outside, through cracks which will not let a bee pass. Entrances should be reduced when robbing may be expected and especially the entrances of weak stocks. The entrances of small nuclei may be reduced to one inch or less.