Saw bee eaters along the entrance track to the Marais du Vigueirat Reserve in June a few years. The reserve itself is very interesting for visiting birders (waders, glossy ibises, moustached warbler, bearded reedling, various herons, terns incl.caspian...).
It is true that you might be a bit early for them but they are recorded each in year in Bouches du Rhône as early as the first decade of April. Listen to the calls, you might come across a migrating group, they're pretty noisy. Check out electric wires too !
Other good sites in Camargue in Spring : the road between Salins de Giraud and Piémanson (beach) passes through a lot of good habitat for terns, slender billed gulls, waders, flamingos (of course !). Don't forget to check the big patches of Salicornia for spectacled warbler in this area (a very good spot for this one is also the Phare de la Gacholle area).
As stated, la Crau is great in spring too and pretty close to the Camargue itself. There it's possible to see calandra lark, southern grey shrike, lesser kestrel, little bustard and pin tailed sandgrouse (also booted and short toed eagles...).
Other places I really enjoyed on my last visit in the region (and I think they are closer to Nîmes than the good spots in Camargue) are Etang du Scamandre (good hides, plenty of birds) and the area around Pont des tourradons (it's a bridge over a large canal, you can walk along the road from the bridge and check the marshes on both sides of the road) had western swamphen and great spotted cuckoo there. Collared pratincole is also frequently reported in that area but I didn't see any on my visit.
Have a nice trip !
