Factors predicting susceptibility of songbirds to nest predation by corvids
A review article by Dr Lucy Capstick
It is not yet clear what the effect of corvids (such as crows, magpies and jays) on songbirds is. Corvids are known to eat the eggs and young of songbirds in the nest but when corvids are removed or when corvid numbers decline thre is not always a positive effect on songbirds.
There are a number of possible explanations for this apparent discrepancy. It may that corvids are unfairly blamed as their behaviour (they are distinctive and active in the day) makes them more easily observed than other predators.
It may also be that other predators, such as mammals or other bird species, take eggs and nestlings when corvids are absent. It could also be that corvids are simply removing individuals that would have died anyway, either in the nest or later in life (overwinter for example).
Finally, it may be that songbird species differ in their vulnerability to nest predation by corvids. Previous research may not have detected an impact of corvids on songbirds because they have included songbirds whose nests are not likely to be predated by corvids.
Songbird nesting biology could influence their susceptibility to predation by corvids. For example, it could be that songbirds with smaller eggs are more vulnerable because corvids can pick up their eggs and take them from the nest.
We explored the latter explanation in this research paper.(1)
We used previous research to find examples of nest predation of songbirds.
We filtered through 5,000 research papers to find the ones which had information we could use.
We only included research that identified different predators taking eggs and/or chicks from the nests.
This meant we could work out how many eggs and chicks were predated by corvids and how many were lost through other predators or other causes of death (such as starvation).
We could then relate the differences rates of corvid predation to the differences in songbirds’ nests.
We found that corvids predated approximately 10% of all nests but that some songbirds were more likely to predated than others.
Songbirds that made open nests (such as blackbirds) as opposed to nesting in holes or in nest boxes (like great tits) were more likely to have eggs or chicks taken by corvids. This is probably because their nests are more accessible.
Songbirds which bred at the same time as magpies and jays were also more vulnerable to nest predation by corvids. This could be because the corvids choose to take eggs and chicks, which are nutrient-rich, when they have their own young to feed.
Using the results of this research we could predict how vulnerable other songbird species are to egg and chick predation by corvids based–– on their nest type and timing of breeding.
We found that there are some examples of songbird species which we predict to be vulnerable to corvid predation (such as blackbirds) responding to changes in the numbers of corvids such as crows and magpies.
However it is important to note that there are many cases when songbirds do not respond to the changes in corvid numbers.
Therefore an understanding of the actions of other predators and the impact of other factors (such as habitat changes) on songbird breeding and populations is also needed.
All factors need to be considered in order to conserve songbird populations.
Read the full paper (1)
Dr Lucy Capstick
Lucy is a post-doctoral research ecologist focusing mainly on agricultural ecology. Her current research is focused on pollinators and crop pollination. In particular, she is examining how flower-rich habitats in agricultural landscapes affect the number of pollinators and the pollination of field bean crops as part of the BEESPOKE project within the Game & Wildlife Conservation Trust.
Previous post-doctoral research has included examining the factors influencing the breeding success of lapwing on arable and wet grassland and looking at the impact of pheasant release on woodland ground flora and woodland ride habitats.
Her PhD project, which was funded by SongBird Survival and the University of Exeter in association with the Game & Wildlife Conservation Trust, looked at the breeding, ranging and foraging behaviour of rural magpies and their effect on songbird productivity.