Melecta, a cleptoparasitic bee

The plum tree started flowering last week and today it was buzzing with bees. I counted six species, Bombus terrestris and lapidarius queens, Anthophora plumipes males and females, Andrena fulva, with males actively patrolling the branches, and the first males Osmia rufa of the year. Later, a black bee with white and grey hair patches and dark wings turned up. It was Melecta albifrons, a cleptoparasite of A. plumipes. I haven't found much information on M. albifrons so the following life history account mainly comes from a study on the American species, Melecta separata. Melecta females parasitise Anthophora species that nest communally. They explore their host's nesting aggregations in search of finished nests. A female, upon finding a nest will start digging and breaking open the sealed entrance. Then she will lay an egg on the roof of the cell, seal the cell and replug the nest. Anthophora females usually attack the cuckoo bee, but she either flies away or if inside the nest it defends herself with her sting. The Melecta larva hatches a day earlier than the Anthophora's and is very mobile. They pierce and drain the Anthophora egg and any other Melecta eggs that she finds in the cell with their long sickle-shaped mandibles. Only one Melecta larvae survives, as if two are born at the same time one will kill the other. The larvae then feeds on the syrupy mixture of pollen and nectar intended for the Anthophora larvae. Subsequent larval stages lack the long mandibles of the first stage. The following year a Melecta will emerge from the cell, having consumed the food intended for Anthophora grubs. In a M. separata nesting aggregation 20% of the nests were parasitized.
The Melecta albifrons visiting my plum showed a very different behaviour from other bees, sluggish, like she didn't want to fly too much, climbing over the flowers to reach each of them and feeding showing a very long tongue. The bee stayed for quite a while feeding on the plum flowers. M. albifrons has a very similar distribution to its host in Britain (click here for distribution map), reaching up to the Yorkshire Wolds in the north. Its peak flight period is a few weeks after the emergence of the host, and flies from April to early June. Given that it doesn't need to collect pollen for provisioning its brood, the bee is not fussy about what flowers to visit, and tends to fly at short daily periods - the warmest - as they are less endothermic than their Anthophora hosts, as shown in the figure below.

References
Thorp, R. (1969). Ecology and Behavior of Melecta separata callura (Hymenoptera: Anthophoridae). American Midland Naturalist, 82 (2) DOI: 10.2307/2423782
P. G. Willmer and G. N. Stone (2004). Behavioral, Ecological, and Physiological Determinants of the Activity Patterns of Bees. Advances in the Study of Behavior, 34 , 347-466 : doi:10.1016/S0065-3454(04)34009-X