Food choice tz

1. Food choice
• Foraging: the act of an animal searching for
food
• Animals seek out food sources that will give
them the most energy reward for the least
amount of energy expended
• Today: examine foraging behaviors of birds in
the quad

2. • What factors might influence seed selec@on in
the birds we will be watching?

3. Birds we might see
Junco Bluejay Nuthatcher
H
H
H
H
Black bird
Sparrow

4. Three types of seeds
• Black oil (soI hulled)
• Safflower (small, soI hulled)
• Striped sunflower (large, thick hulled)

5. • Come up with a hypothesis as to why birds
will choose or not choose certain seeds.
• What is your alterna@ve hypothesis?
• Would your hypothesis change with different
birds?
• If your hypothesis is correct, what do you
expect to observe
at the feeders?

6. Variables
•What is the independent variable?
– Manipulated
•What is the dependent
variable?
– Measured

7. Observa@ons: 20 minutes
• Try to be quiet and s@ll
• What species of birds appear?
• What types of seeds does each species eat?
• How long does it take an individual bird to eat a
seed and return to the feeder?
• Do different species of birds have different seed
handling techniques?
• Record all of your data!

8. Data Analysis: Chi2 Test
• Sta@s@cal test to test our bird feeding hypothesis
• Test based on differences between the observed
results and the expected values (based on null
hypothesis)
• The formula for X2 is as follows:
– o is the observed frequency
– e is the frequency expected under the null hypothesis of
no difference between groups.

9. Example: Snail sediment type preference
• 20 snails
• 2 sediments: mud and sand
Null hypothesis: There is no difference in
sediment preference for this species of snail.
• Expected results: 10 in mud, 10 in sand
• Actual results:
– Trial 1: 13 in mud, 7 in sand
– Trial 2: 11 in mud, 9 in sand

10. Chi2 Test on snail results
Expected value Mud: (24 x 20)/40= 12
Expected value sand (16 x 20)/40= 8
Chi2= [(13f12)2/12] + [(11f12)2/12]+ [(7f8)2/8] + [(9f8)2/8]= .416
•Now you compare your experimental value (0.416)
to a cri@cal chi2 value.
Mud Sand Total
Observed 1 13 7 20
Observed 2 11 9 20
Total 24 16 40

11. Chi2 Test comparison
• Degrees of Freedom: the number of independent
cases
(Mf1)(Nf1)
M= # rows N=# columns
(3f1)(3f1)=4
• p level: level at which the given sta@s@c is on the
border between rejec@ng or not the null hypothesis
– Probability that values outside this set level are due to
chance sampling errors rather than real differences
• Use p=0.05
– At p=0.05 you are 95% sure your results are real

12. Chi2 Table
• If calculated>
cri@cal value @ p
level, reject null
hypothesis
• If calculated<
cri@cal value @
alpha level, fail to
reject null
hypothesis