It's been previously suggested that learning may play a role in artificial nest predation experiments, as Yahner et al. posited. Where eggs are replaced daily in fixed nest sites, particularly in ground locations, there is the potential for predators to learn the locations, particularly if there are olfactory or visual cues associated with the nests. Intelligence and learning in nest predators may be an avenue for future study, to establish existing predator behaviour and the usefulness of fixed-location artificial nest experiments in general.
Predation intensity on arboreal versus ground nests
Predation intensity at ground levels were significantly larger than those in arboreal nests, which supports previous studies \cite{article,Piper_2004}, although this is not universal \cite{articlea}. Nest height has been previously hypothesised an important survival factor for nests, due to the larger diversity of terrestrial opportunistic predators \cite{Martin_1988}.
One of the more unusual results from this experiment is the spatial autocorrelation between arboreal nest proximity and location, which is not present on ground nests. Particularly given the dispersal constraints of terrestrial predators are usually larger than those of arboreal predators. This may be due to the density of foliage in particular areas, making it easier to access adjacent trees. While areas with a sparser density will not allow predators to traverse from one tree to the next. This could be an avenue of future study, however it should be noted the spatial autocorrelation was quite weak (r = 0.0889), so the result may not be particularly ecologically significant.
3. Predation is very very high here compared to other studies, why is that?
Nest predation rates in this experiment is very high compared to other artificial nest experiments.
One possible mechanism for this is the mesopredator release effect, whereby in the absence of apex predators, mid-ranking predators take over the role of apex predators in the food web \cite{SOULE_1988}. In unfragmented habitats, apex predators can exert top down ecological control of an ecosystem, which has indirect benefits on prey populations by suppressing mid-level predators \cite{Elmhagen_2007}. However, larger species are more vulnerable to habitat fragmentation due to the larger range size, meaning mesopredator release may become a growing problem for an increasingly fragmented U.K. forest habitat \cite{Prugh_2009}. This in turns suggests that apex predator control measures may be a valid conservation mechanism for ground nesting bird populations through top-down control of rodent populations. A possible future route of study in this area should aim to examine the populations of apex predators and mesopredators in this woodland, and examine whether the mesopredator release hypothesis is a good fit for these findings.
Implications for bird conservation in the U.K.
Nest predation is the most common cause of nest failure, and previous experiments in the U.S. have found over 75% of ground nest predations are caused by terrestrial rodents \cite{Sanders_2019}. These are slightly higher than our experiment 63%, and may be accounted for by the presence of higher level predators not present in the U.S., such as bagders and foxes.
1. How many ground nesting birds live in the U.K.? What proportion are endangered and live in deciduous woodlands?
Potential drawbacks
One of the potential limitations of this experiment is the use of artificial nests over natural ones, which attract different predators. Previous experiments have shown artificial nests can overestimate predation rates, by attracting different predators \cite{WEIDINGER_2008}. In addition, artificial nests do not mimic the seasonal variation in predation rates observed in natural nest experiments \cite{Zanette_2002}.
The cryptic nature of artificial nests may provide a source of bias during fixed nest experiments, especially if there are associated sensory cues. While olfactory cues from the plasticine eggs in this experiment was controlled for, the artificial nests were made from non-native conifer leaves, which may have provided cues to predators.
This experiment was also short lived, having only been run for 2 weeks post spring, which only provides a brief insight into the nature of the edge of this forest. Future studies should aim to control for seasonality by performing longer term experiments, especially when orientation plays a factor in the abiotic conditions of the edge.
Acknowledgements
I would like to thank Cristina Banks-Leite of Imperial College London for help and advice regarding the pilot study, statistical analysis and invaluable guidance. I would also like to thank the technical staff at Silwood Park for their assistance using the GPS unit for spatial analysis. Many thanks to the Biology Undergraduate department, without whom this project could not be possible.