FORAGING (PART II). Foraging Decisions Feeding holes (24/log) -seeds placed in 0, 6, 12, and 24...

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