The experimental provision of calcium increases the reproductive success of passerine birds

The role of the availability of calcium in limiting the breeding success and intensity of eggshell specking in birds remains unclear. Experimentally, we provided blue tits (Cyanistes caeruleus) and great tits (Parus major) in woodlands with underlying calcium-rich limestone or calcium-poor gritstone with calcium during the egg stage or during the entire reproductive period and examined their breeding success and eggshell speckling. When provided with calcium during the egg stage, experimental blue tits laid larger clutch sizes with higher clutch weights than controls, whilst experimental great tits had higher fledging success than controls. When provided with calcium throughout reproduction, there was no effect of treatment on blue tit reproductive success, but experimental great tits had higher hatching success and nestling masses than controls. Additional calcium during the egg stage resulted in experimental great tits, but not blue tits, laying less speckled eggs than controls, and additional calcium throughout reproduction led to experimental blue tits and great tits laying less speckled eggs than controls. The provision of calcium led to both species generally having higher levels of reproductive success and less speckled eggs than controls, irrespective of habitat geology, meaning that calcium availability does determine avian breeding success and eggshell specking.