L18-vision and diving for birds Flashcards
Amphibious trade-offs
Antagonistic requirements of thermoregulation and
buoyancy
The cormorant strategy: low buoyancy, high heat loss…
energy expenditure cormorants?
very high efficiency
very high success rate
Global climate change Good for cormorants?
can’t feed in see ice
need ice free areas
can cope with cold waters
climate change may be good for cormorants
The cormorant paradox:
A tropical bird in the arctic
• Great cormorants – Partly wettable plumage – Low buoyancy – High thermal conductance – Breed north of the Arctic circle at 69ºN in Greenland – Dive in water below -1 ºC
One idea to explain paradox is extreme lifestyle On the edge of starvation what is the evidence?
On the edge of starvation?
• Low resting and foraging heart rates and
body temperatures
• Low fat content (ca 4%)
Are Greenland cormorants on
the edge of starvation?questions to think about?
- Are they lean, compared to other birds?
- Where do they go during winter?
- What are they doing, and what does it cost?
- How do they see what they are doing?
Body composition
4% is low for humans
not so for birds most birds are lean
cormorants are typical for birds their size
most species above them only are so for short periods of time
not likely starving
Geolocation Loggers results?
can work out how far north or sunset by length of time
found that cormorants are moving further south of arctic circle during winter
Temperature
body temp changes in april
Circannual variation in heart rate
in depth of winter will forage at night
birds rearrange the day by crossing arctic circle
moving to more favourable conditions
pre return to colony in march/april peak in foraging in morning
post return feeding towards the end of the day
Summary
• Disko cormorants winter throughout Greenland,
and dive in very cold water
• Because they use an energy saving strategy of
reduced buoyancy, diving in cold water
compromises thermoregulation, and body
temperature is related to water temperature
• Despite the potentially high costs of rewarming,
there is no evidence that they have to pay for it
• Field metabolic rate is not particularly high
• But foraging yield is….
Are cormorants able to minimise the time spent in water, and therefore forage cheaply, as a result of sensory adaptations for their amphibious habit?
eye guided pursuit predators
eyes struggle in water as refractive index cornea is not similar to air but is similar to water meaning different amount of refraction so blurry image
How to train cormorants?
one stripe orientation will give fish
point of no return
if can’t tell will just guess so 50% chance
Visual acuity of great cormorants?
vision is pretty rubbish
human without goggles can see just as well underwater
especially bad with lower light
small region binocular but wide range of vision around them
Great cormorants: fantastic mediocrity
• Field metabolic rate is similar to that predicted by body mass
• Wettable plumage is not a thermoregulatory liability in cold water
• Body fat similar to other species
• Not ‘on the edge of starvation’ at the end
of winter
• Visual acuity is similar to humans
