Species with unusual mating systems and sex role reversal are valuable for testing theories about sexual selection and the evolution of reproductive traits that have been developed using more typical species. We used the polyandrous red-necked phalarope to test predictions about biases in primary sex ratio, factors influencing uniparental incubation and the evolution of small egg size in multi-clutching shorebird species. Egg density differed with embryo sex, and despite females being larger as adults, egg size increased with male-biased clutch sex ratio. Males had higher incubation attentiveness with greater body mass and in warmer, drier weather, while early nest initiation and increased incubation load decreased attentiveness. We detected no direct effect of experimentally manipulated incubation load on behaviour, however a greater incubation load increased the probability of nest abandonment. Our results highlight the effect of environmental conditions on reproduction in this species, and the need to reevalute our predictions about the effect of polyandrous breeding systems on reproductive traits
