The shelf life of bird eggs: Testing egg viability using a tropical climate gradient

Avian parents influence the onset of development, hatching synchrony, and likelihood of brood reduction through the onset of incubation. Studies testing adaptive functions of brood reduction assume that eggs are protected by their shells and waiting for parents to initiate incubation in a manner that creates optimal hatching patterns, but the viability of early laid eggs may diminish over time due to putative effects of ambient temperature. We tested effects of exposure to ambient temperature on the viability of unincubated bird eggs using an altitudinal climate gradient in Puerto Rico: daily maximum temperatures nearly always exceed developmental zero in the lowland forest, whereas temperatures rarely exceed developmental zero in the cloud forest and at intermediate altitudes. We removed 382 freshly laid Pearly-eyed Thrasher (Margarops fuscatus) eggs, placed them in holding boxes at three sites for 1-7 days, and returned them to nests to be incubated. Additional control eggs (n = 39) tested effects of both handling and movement on hatching success, and unmanipulated eggs (n = 139) provided a measure of natural hatching success. Hatching success of control eggs (82.0%) and unmanipulated eggs (84.9%) did not differ, indicating no impacts of handling on hatchability. Hatching success of experimental eggs exposed for one day (78.6%) was high but declined very strongly after exposure for three days (41.9%), five days (11.5%) and seven days (2.1%). Hatchability of eggs held at the cloud forest did not differ from eggs held at the lowland site but was lower for eggs exposed at mid-elevation. Hatching success was positively related to minimum temperature and marginally related to mean temperature and proportion of time above developmental zero. Exposure duration, treatment site, and maximum temperature were the only significant effects identified by stepwise logistic regression. Eggs held at the lowland site had shorter developmental periods than eggs held at other sites. Most embryo mortality (80.8%) occurred at very early stages. Our results demonstrate that viability of unincubated eggs exposed to moist tropical conditions declines strongly but suggest that ambient temperature is not the sole cause. We compare rates of egg viability decline among species, examine alternative mechanisms for the loss of viability, and discuss the significance of egg viability on avian life histories.