Bryan
Hummel
Ecology 3434
Ribble;
8:30 TR
February
16, 1999.
Bird Habitat Use In Brackenridge Park
Introduction:
A study of birds was conducted in
Brackenridge Park in order to get a better understanding of the types of
habitat used by the different types of birds. It was expected that birds would
be found on different trees and in different places according to their
preference. The study showed that one
hypothesis was true while the other was false (now
doesn’t this get your attention and push you to read further to find which one
is true), and its
results are discussed below taking into account the limitations of the study.
Methods:
Five
different plots were simultaneously surveyed in Brackenridge Park to determine
which types of trees were favored by different types of birds according to
their size. The birds were divided by
size into three categories: Large (Cardinal size or larger), Medium (Sparrow
size), and Small (Warbler or Vireo size).
The size of the bird landing in each of the plots was recorded as well
as on what part of which species of tree they landed on. Each tree was divided into three sections:
Trunk, Inner Branches, and Outer Branches.
The data was compiled into two tables and two “null” hypotheses were
made in order to compare the findings of the survey with the “null” idea that
the birds would show no preference for species or placement. The first table is
based on the species of trees and the number of birds landing on those
particular trees. The second graph was
comparing the size of birds to the part of the tree that they landed on.
Results:
|
Tree
Species |
Number |
Rel.Density |
Observations |
Expected |
Chi-square |
|
Cedar Elm |
92 |
0.308724832 |
51 |
27.785235 |
19.396104 |
|
Escarpment
Cherry |
7 |
0.023489933 |
0 |
2.114094 |
2.114094 |
|
Ligustrum |
75 |
0.251677852 |
9 |
22.651007 |
8.2270067 |
|
Live Oak |
12 |
0.040268456 |
6 |
3.6241611 |
1.5574944 |
|
Percimin |
3 |
0.010067114 |
0 |
0.9060403 |
0.9060403 |
|
Texas Oak |
5 |
0.016778523 |
0 |
1.5100671 |
1.5100671 |
|
Shrub |
104 |
0.348993289 |
24 |
31.409396 |
1.7478575 |
|
|
|
|
|
|
|
|
Totals |
298 |
1 |
90 |
90 |
35.458664 |
Table1.
The species of tree is listed in column 1
and the relative density, number of bird observations, expected number of bird
observations, and chi-square (c2) value were calculated for each species. The rows of the highlighted boxes should be
combined into one row because the expected values are less than three.
|
Part Of
The Tree |
Small |
Medium |
Large |
Total |
|
Trunk |
|
|
|
|
|
Observed |
2 |
4 |
8 |
14 |
|
Expected |
2.5452 |
2.7573 |
8.6961 |
13.9986 |
|
Chi-square |
0.1167857 |
0.56007808 |
0.05572098 |
0.7325848 |
|
Inner
Branches |
|
|
|
|
|
Observed |
8 |
5 |
14 |
27 |
|
Expected |
4.90908 |
5.31817 |
16.77269 |
26.99994 |
|
Chi-square |
1.946146 |
0.019035147 |
0.45835282 |
2.423534 |
|
Outer
Branches |
|
|
|
|
|
Observed |
2 |
4 |
19 |
25 |
|
Expected |
4.545444 |
4.924231 |
15.530267 |
24.999942 |
|
Chi-square |
1.425446 |
0.173469307 |
0.77519898 |
2.3741143 |
|
|
|
|
|
|
|
Total
Birds |
12 |
13 |
41 |
66 |
|
Total
chi-squares |
3.4883778 |
0.752582535 |
1.28927278 |
5.5302331 |
Table 2.
The trees are divided into three
sections: trunk, outer branches, and inner branches. For each of these three sections, a row was constructed for the
number observed, expected number of bird observations, and chi-square (c2) value. There are
three columns according to size: large, medium and small.
The results of the two tables were
essentially that birds did show a preference for different species of trees,
but not necessarily for their placement in those trees. The null hypotheses were accepted or
rejected by comparing the calculated chi-squared values with a table of c2 values in relation to the degrees of freedom. The null hypothesis can be rejected if the P
value (taken off the c2 chart) is less than 0.05.
Table 1 compares the numbers of
trees by species in the observation area and the number of birds using each
species of tree. The chi-square values
for each tree added up to 35.46 and the table had 4 degrees of freedom (# of
rows – 1). When finding a P value using
the table of c2 values, it was found that P << .005, thus the birds
were not selecting different tree species at random. Table 2 compares the part of the tree used with the size of the
bird using the tree. The c2 values were totaled for the small, medium, and large birds
in each area of the tree. These totals
for trunk, inner branches, and outer branches were then totaled to get an
overall total c2 value.
The final value was not large enough to reject the null hypothesis, so
it cannot be said that the birds are not choosing the area of the trees at
random (c2 = 5.53; P<0.5).
Discussion:
The results may at first seem
puzzling. Why would the birds associate
with specific species of tree, but not with the parts of that species? I will try and explain the results from two
different approaches. First, assuming
that birds are choosing the species of tree because it contains a resource they
like (namely food items) it seems feasible that the birds would show preference
for the trees containing the resource, which is what Table 1 concludes. It would also seem logical at first that the
resource (seeds or fruit) would be unevenly distributed throughout the tree and
thus the birds would show preference for certain parts of the tree. Table 2 does not conclude that there is an
association, so an alternate logic must be used to support the data. Lets assume that the resource the birds are
using is not produced by the tree, but rather it is also on the tree and it has
no association with the parts of the tree (ex. a worm that lives on the bark of
the tree). This would allow the bird
access to the resource on any part of the tree, and thus the data would show no
definitive association between the birds and different parts of the tree.
One also might expect predation to
play a role in the areas used by the various birds even though it counters the
data. If predation were prevalent, one
would expect the birds to land on the part of the tree that afforded them the
most protection (i.e. the inner branches) yet this was not supported by the
data. To support the data, I would
conclude that predation is not a major factor on where the birds land in the
trees. The lack of predators, namely
hawks and other birds of prey, is one way to explain a low level of
predation. This assumed lack of
predators might be caused by the location of the observational area. Brackenridge Park is located near downtown
San Antonio, which is not a very natural place for a hawk to live. If the number of predators were lowered
because of location or proximity to an undesirable predator habitat, one would
expect the effect on the birds being studied would be lessened. If there were so few predators that they did
not pose a real threat, then the birds would not select a part of the tree
because of protection from the predators.
This logic would also support the data of Table 2. I am sure that there are several (even
hundreds) of alternate explanations for the results that were obtained, but I
just outlined two that could possibly explain the results. A much larger study or the same area over a
much longer time would take out a significant amount of possible error and may
also change the outcome of the data, but there was not enough space or time
allotted for further study.
Bryan Hummel Ecology 3434
February 16, 1999.