Measuring Eastern Bluebird Embryo Heart Rate in the Field Using a Digital Egg Monitor

Measuring embryonic development inside the egg can provide useful information for research on the breeding biology of birds. Because embryo heart rate is correlated with oxygen consumption, it provides a way to quantify embryo metabolism as a proxy for development. However, it can be difficult to non-invasively measure embryo heart rate in the field. We used a relatively new technology, the Buddy digital egg monitor, to measure the heart rates of Eastern Bluebird (Sialia sialis) embryos. We wanted to determine how well this machine would work for field studies and to test whether embryo age, external egg temperature, or clutch size were correlated with embryo heart rate. We found that heart rate significantly increased as the age of the embryo increased and as the external temperature of the egg increased. However, there was no relationship between heart rate and clutch size, suggesting that even females with larger clutches are able to maintain their nest temperature at a level that supports embryo development to produce viable young. Although we were able to obtain useful data that addressed our study questions, using this monitor presented some challenges. We frequently could not obtain a heart rate reading when the embryo was moving too much (36% of readings) and sometimes no heart rate was detected (33% of readings), even in eggs that did eventually hatch. Overall, we found that this device has the potential to be useful in a wide variety of field studies, but that researchers should be prepared for an approximately two-thirds decrease in sample size due to unsuccessful measurements, as well as potential bias introduced by the failure to obtain a reading from moving embryos.