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FORAGING (PART II)
Foraging Decisions
Feeding holes(24/log)
-seeds placed in 0, 6, 12, and 24 holes
After a bird samples a log, how does it decide to stay or move on?
Currency: maximize rate of intake
Foraging Decisions
After a bird samples a log, how does it decide to stay or move on?
If 0 or 24 holes with seeds- - average of 1.7 ‘looks’
If 6 or 12 holes with seeds- Model predicts 3 or 6 ‘looks’ per log
Experiments – 3.5 and 6.3 ‘looks’ per log
Fixed portion – 10 nuggets/day
Variable portion – 5 or 20 nuggets/day
WHICH IS THE BETTER OPTION?
“BETTER” depends on benefit of eating different numbers of nuggets
If need 10 nuggets to survive – should use 10 nugget option
If need >10 nuggets to survive – lose nothing by going to 5/20 option
Therefore – animals should be sensitive to both mean rate of return AND variability
Caraco et al (1980’s – 1990’s)
Juncos - Junco phaenotus
Feeders
Every visit
OR
NOTE: Same average reward
Constant reward
Variable reward
Caraco et al (1980’s – 1990’s)
Juncos - Junco phaenotus
Feeders
Every visit
OR
Juncos behave as if they are risk adverse
Caraco et al (1980’s – 1990’s)
Juncos - Junco phaenotus
OR
Second question: Is there a level of food at which juncos start to become risk prone?
Add food to variable feeder
<
Reward = 3 Average reward = 6
Caraco et al (1980’s – 1990’s)
Juncos - Junco phaenotus
OR
When Reward constant = ½ Reward variable
50% of juncos chose the variable
Caraco et al (1980’s – 1990’s)
Did experiment at 1 and 19ºC
Feeders
OR
Constant reward
Variable reward
Adequate at 19ºC but not at 1ºC
Option preferred at 1ºC
A MODEL FOR THIS
1
0
5 6 78 9 10
Survival
Energy level at dusk (arbitrary units)
A MODEL FOR THIS
Foraging options:
1) Provides 1 unit of energy with a probability of 1.0
2) Provides 2 units of energy with a probability of 0.5 and 0 units of energy with a probability of 0.5
Option chosen depends on current hunger state
Energy level if choosing:
Best to chooseState (i) (ii)
6
7
7
8
8 or 6
9 or 7
(ii) – take risk
(i) – play it safe
FOOD STORAGE
Clark’s nutcracker- can store about 30,000 seeds in 2500 – 4000 locations
Paridae - can store between 100,000 and 500,000 seeds
FOOD STORAGE
Imagine two strategies
Hoarder Freeloader
Cost - high
Benefit - high
Cost - low
Benefit - high
AN ESS ONLY IF MEMORY IS INVOLVED
Willow tit
Seeds labelled with 35S
Number with labelled feathers
Given 35S seeds
Not given 35S seeds
Optic chiasma
Time Visits
Per
cen
tage
Same eye
Other eye
Marsh tit
Clark’s nucracker
Memory in Corvids
Pinyon jay
Mexican jay
Western scrub jay
High
Low
Reliance on stored food
High
Low
Spatial memory
Hippocampal size
Groups that don’t store food Groups that store food
Hippocampal size
Shorter daylength
Milder winter
Cognition
Western scrub jay
Store nuts
Store nuts
Store worms
Store worms
Retrieve
Retrieve
0
0
120
120
124
124
PREFER NUTS
PREFER WORMS
Cognition
Western scrub jay
Food delivered when lights come
on
Food delivered 2 hours after lights
come on
Cognition – variety in the diet
Western scrub jay
Fed nuts Fed dog food
Feeding and vigilance
Feeding and vigilance
Heller & Milinski, 1979
Feeding and vigilance