Most mums are determined to give their young the best start in life, and mother ducks are no exception. Settling down to incubate her eggs, an expectant mum will only move if it is absolutely necessary. However, even in the best case, this often means abandoning her eggs to feed for two brief periods a day. And stressed mothers may have to leave even more. According to William Hopkins, from Virginia Polytechnic Institute and State University, ducklings incubated by mums that take longer or more frequent breaks from the nest pay a price; they hatch with high levels of stress hormones. But without a direct route of communication between the mothers and their young, it wasn't clear why the ducklings were stressed and what affect it may have on their development. Then Hopkins' student, Sarah DuRant, had an idea: maybe the mother's brief absences were lowering the eggs' incubation temperatures and that was causing the ducklings stress. The duo decided to find out what effect low temperatures had on duckling stress and their development (p. 45).
Hopkins and DuRant teamed up with duck expert Gary Hepp from Auburn University to collect freshly laid individual eggs from wild birds' nests before returning to Virginia Tech where they could be incubated. ‘Gary had published a study that demonstrated that very small changes in incubation temperature could affect the body size and composition of the hatchlings,’ explains Hopkins, so the team opted to incubate the eggs at 35, 35.9 and 37°C, all within the normal temperature range, and waited for them to hatch.
The first thing that the team noticed was that the coolest eggs took longer to develop. ‘At low temperatures they take 37 days and at higher temperatures they take 31 days,’ says Hopkins, also the hatchlings looked perfectly normal, regardless of their incubation temperature, so the team moved the ducklings together into a room and monitored their development and stress hormone (corticosterone) levels.
But within a few days, things started to go wrong for the cold incubated ducklings. Hopkins and DuRant carefully collected blood samples from the ducklings, analysed their corticosterone levels with Ignacio Moore and found that the resting cold incubated ducklings had higher stress hormone levels then the ducklings incubated at warmer temperatures. And when they tested the ducklings' reactions to stress they found that the cold incubated ducklings' hormone levels rocketed compared with those of the ducklings incubated at warmer temperatures. Incubation at a cooler temperature had increased the ducklings' stress hormone levels, but had this also affected other aspects of their biology?
Monitoring the youngsters' growth, the team realised that the cold incubated ducklings failed to thrive. They grew slower than ducklings incubated at warmer temperatures and, even more surprisingly, the cold incubated ducklings began to die 4 or 5 days after hatching. ‘Normally most mortality occurs in the first couple of days after hatching,’ explains Hopkins, ‘but in the cold incubated chicks 75% of their mortality is after days 4 and 5, which suggests that there is a latent effect. Something is not right physiologically and causes the birds to fail to thrive’.
Hopkins speculates that lower incubation temperatures delay the ducklings' development and this could prevent them from thriving. He also adds that this discovery could have an impact on avian ecologists. ‘What is interesting to me is that avian ecologists in the field look at hatching and fledging success, but that does not necessarily mean that you have a good understanding of the quality of the offspring. For that you need to monitor the young birds longer,’ says Hopkins.