Your article examines an interesting point about the role of sentinels. However, if nest predation of incubating females is extremely low, it seems difficult to support your premise. Also, how similar is the reaction of sentinels to humans as it is to potential nest predators? Do you have any evidence that birds respond to potential predators in the same way as they do to people? That is, are people the appropriate experimental model? Also, you did not describe the behavior the putative sentinel carried out to warn the incubating bird. Vocalization? Movement? Finally, you mentioned that the incubating bird did not necessarily see the approaching human, but you also stated that the nest is somewhat flimsy and I would suspect that the birds can see through the nest.
Hi James, Thanks for your question, which appears to compromise four sub questions. First, 9 of the 205 nests (4%) were predated, which agrees with a larger study that showed that whilst the predation rate of zebra finches in nestboxes is low (about 2%), levels of predation can be as high as 60% in natural nests (Griffith et al, 2008, Emu, 108, 311-319). So, whilst the sentinel birds in our study of nestbox-breeding birds may not have been under strong selection against predators, such sentinel behaviour is likely to be under strong selection as the birds have only recently begun nesting in nestboxes within evolutionary time periods. Second, we accept that a human predator is unlikely to be entirely representative of a non-human predator, but we used a human predator in order to standardise the day on which the potential threat was presented. Also, the use of non-human predators would have been logistically very challenging at our field site. Whilst we are sure that you can understand why we chose to take this approach, we do accept that another study that examines non-human predators would be informative. Third, we do not describe the behaviour by which the sentinel birds warned the incubating birds of the approaching predator because we are unsure of the exact behaviour. Nevertheless, we outline the possible behaviours in the sixth paragraph in the discussion and suspect that vocal contact between the sentinel bird and the incubating bird causes the incubating bird to leave the nest when a predator approaches (e.g. Elie et al, 2010, Animal Behaviour, 80, 597-605). Fourth, there has been a misunderstanding regarding the disparity between the incubating bird not seeing the approaching human and the nests being flimsy, and we apologise for any part that we have played in it. The experiment was performed on birds nesting inside nestboxes, with small entrance holes at the front, and so when the human observer approached from the side of the nestbox, there was no way that the incubating bird could see the approaching human as the wall of the nestbox would have obscured their view. When we refer to the nests being flimsy, we are referring to the nests that are built within the nestboxes. We hope that you find these answers informative. Best wishes, Mark Mainwaring and Simon Griffith.
Thanks for the clarifications. I respect that this kind of experimental analysis is not the easiest thing to do and we often have to work with what we can as best we can. Cheers. Jim
- James Roper •