Hello, i'm Dr Paul Rose and in this short video I'd like to
explain some of the links between feeding and foraging behaviours
and anti-predatory behaviours, how animals balance the need to collect
food and energy against energy expenditure on vigilance
behaviour.
I'm going to mainly use the European
starling as my example, but a few other species will be used as well.
So here is the European starling, Sturnus vulgaris,
a common garden bird in the United Kingdom,
this animal is feeding on a high energy
resource, some fat balls that have been put out on
a bird feeder, this is an important food source for
this bird which is versatile and very generalist
in its foraging strategies.
Because the starling is so generalist it
has become very successful, it can exploit a range of different
habitats and exist in many different environments around the world
consequently it has been able to colonise non-native areas
particularly thanks to the behaviours of human beings, and you can now
find starlings in many countries that they would not normally be found in.
Because starlings are omnivorous they utilise a wide range of feeding and
foraging behaviors both on and off the ground this means
they can consume a wide range of different food resources
hence their generalist nature this is in exact contrast to highly specialized
animals such as the giant panda, which is an
extreme habitat specialist, only feeding on bamboo, and consequently
has a restricted range and habitat choice.
Let's talk a little more about foraging behaviour,
a food patch is the area where an animal is feeding
and within that food patch animals try to feed optimally
i.e they like to gain as much energy as possible from the feeding patch
whilst expending as little energy as they can
on searching and processing behaviours for that particular resource that they
are accessing.
For animals to forage optimally,
they must assess the quality of the food resource
and the quality of the foraging patch that the food is contained within.
Therefore animals can use information from
others to determine when to move on from foraging,
where is everyone else feeding that might be good value,
and what is the most cost effective foraging behaviour
within that feeding patch.
Because starlings commonly forage in groups much research has been conducted
on how they use local information to work out the most effective feeding and
foraging strategy for that habitat.
Starlings are able to
use information from conspecifics, i.e birds around them, to work out where
food resources are most plentiful.
Remember food resources are highly valuable but they are
often found in very limited areas and in limited numbers,
and lots of food resources cause effort on the part of the animal trying to
access the food resource such as in these foraging butterflies
that are having to flap their wings whilst they hold on to this flower to
insert their proboscis to gain the nectar.
That is a very costly behaviour which is balanced against the energy
that they can get from the food itself.
One of the things that can interfere
with optimal foraging is the performance of other behaviours
that the animal has to do around foraging, for example
care for youngsters as you can see in this video
of an adult styling with the black spotty plumage
having to deal with three very hungry youngsters that also want their dinner.
Therefore not only is the animal having to work hard
to ensure it can provide for its own energetic needs
it's also having to work hard to meet the energy demands of its youngsters too
who are less effective and less efficient
at foraging until they can learn how to forage more appropriately.
Youngsters also bring with them other challenges,
they are more likely to be attacked by predators
and consequently adult birds will increase their vigilance
rate to ensure that the flock is remaining
wary of any potential danger. So there is a higher cost
on the adult birds whilst they are trying to forage
when youngsters are around both energetically
as well as from a vigilance point of view to look out for danger.
Here is a group of starlings foraging on a lawn
what you might be able to see is the vigilance behaviour
that occurs whilst the animals are feeding
the birds put their heads up and scan the immediate environment
the adult birds, those with the black plumage do this more
than the youngsters, those with the brown plumage
whilst I was filming this I was lucky enough to see
the escape behaviours of the birds, they all fly off
when they sense the predator.
And here is my predator,
I hope you can hear its calls, it's a carrion crow,
this crow is particularly dangerous to young birds.
All the starlings have moved within this flock
to a new safer feeding area under some larger trees,
the birds continue doing the scanning and the vigilance behaviours
around their foraging activities and again it is the adult birds
that perform this more than the youngsters.
Let's look at some of the vigilance costs associated with foraging,
this paper by Fernandez-Juricic et al suggests that starlings have time
competition between foraging and scanning, this means
that birds are constantly having to determine
which is the best and most optimum behaviour to perform
in that environment. Do I look for food or do I scan for danger?
Interestingly when social information, i.e. what birds can glean from each
other's presence is reduced, and this may be because of
visual barriers between conspecifics, the starlings have to spend more time
scanning, they have to do this to find out what everyone else is up to,
in more open environments birds don't need to do this as much
because it's clear to everybody that they can see what's going on.
The neighbourhood distance and the visual obstruction rates
negatively affect food searching, consequently where your
neighbour is and how much of a barrier there is
between birds is going to reduce the amount of food you intake because
you're going to have to look more closely
to see where everyone is and what everyone is doing.
This vigilance scanning is strongly connected to the presence of other flock members
and as I said previously the presence of youngsters in a group
also adds a vigilance burden.
In the case of other species it's not
necessarily just predators that impact on foraging
and vigilance time, in the case of research on giraffes
the type of animal within the group, male or female, young or old, has more of
an impact on the scanning and vigilance
levels than the potential presence of predators.
Even though a baseline scanning for predators
is still maintained.
When giraffes are foraging together
I appreciate these are in captivity, the composition of the group is more
important than the likelihood of predators being
nearby.
Energetic costs are imposed on giraffe
when they have a "high head up" vigilance position
which affects the amount of time they will spend performing that behaviour.
And here's what i mean by that "high head up" position
this is a female giraffe with her calf and she is spending more time performing
vigilance than she would normally.
So I hope you can see
from this presentation, that optimal foraging is key to how
animals gain sufficient energy,
but they can only forage optimally if they consider the performance of
other important behaviours.
The social composition of their groups,
the presence of youngsters, will affect vigilance rates,
these vigilance rates are often but not always
associated with the presence of predators.
Much work has been performed on the group
effects of foraging, you can find very many
interesting references on how animals forage
optimally in groups and how they balance the need
for energy requirements against the looking out for and scanning events
for predators.
Animals that forage in groups are
constantly considering where and how they should feed, as well
as the best strategies to employ to avoid predation.
you