Woodpecker Hole Lab - Ecology & Evolutionary Biology



ECOL 487L, Animal Behavior Lab

Lab#9: Habitat selection- woodpecker nests

This week, we will be looking at the habitat selection in animals. We will meet at Koffler, and distribute to different locations on campus to find woodpecker nests and study their orientation.

Background

Habitat choice is one of the most critical aspects of animal behavior. Properties of habitats animals live in influence many of their behaviors and life history traits, including reproduction and survival. Choice of habitat, in turn, can be influenced by various aspects, such as territoriality and body size.

An important component of habitat choice is nest site selection. Animals have to take care of not only themselves, but also their offspring that are more vulnerable to environmental conditions than the adults are. The conditions inside the nests can significantly influence the behavior of adults (such as time and energy spent on incubating) and survival of the offspring, including the viability of the eggs (Ardia et al. 2006). For example, several bird groups nest in cavities they excavate on trees. The orientation of these cavities, as well as their physical location, may influence the temperature and wind the nest receives.

In climates where temperature levels fluctuate highly during the day, a nest facing south could provide more thermal protection. In contrast, if the temperatures reach high levels during the day, keeping the nest cool becomes more critical, and the nests may be expected to face north to benefit from the prevailing winds (summarized in Zwartjes and Nordell 1998). Other factors that can influence the nest orientation are whether the entrance is close to other branches (the costs of this type of placement has been studied, but can you think of any benefits?), or whether the nest is on the body versus the branch or arm. The plant that the nest is in can be very important too- not only because some plants such as cacti and palm trees are much easier to excavate, but also because cactus tissue has larger heat capacity and therefore produces more heat stress than wood tissue (Howe et al 1987).

On campus

The main tree excavators on campus are the gilded flickers and the gila woodpeckers. Gilded flickers are larger (about 28 cm) than the Gila woodpeckers (24 cm). Gilded flicker males have a red “mustache”, but the females lack this red marking. In contrast, the Gila woodpeckers males have a red cap on their head (this distinguishing feature is absent in the females). However, the females (and males) of Gilded flickers can be distinguished from Gila woodpeckers by the presence of numerous black spots on their belly and chest. For more information, see Moore (1995) and Edwards&Schnell (2000).

Both species nest in the cavities they excavate in saguaros and other trees, but they do not line their nests with fibers or soft plant material. As a general rule, Gilded flickers nest higher (within 3 m from top) on saguaros due to their larger (7-10cm) cavity requirements, while Gila woodpeckers nest lower on saguaros with smaller cavities (4.5-6cm).

Saguaros are one of the plants that require other trees (nurse trees) in order to survive their first few years. These nurse trees, which usually include palo verde and mesquite, protect the seedlings and the young saguaros from intense sun, cold, and animals. Saguaros grow very slowly, it can take 75 years or more for the first branch (arm) to appear. Keep in mind that saguaros have a relatively simple morphological structure with only a few branches (Zwartjes and Nordell 1998), so how would our data collection and results be influenced if we were to focus on another plant that has a more complicated branching pattern?

Instructions

Please bring a calculator to lab with trigonometric functions on it!

Divide into four groups. Before leaving the building to collect data on woodpecker holes, take about 5 minutes to think about what variables may be important to woodpeckers when choosing habitats to nest in. For instance, which of the following may be important, and why: nest orientation (north, south, etc.), nest height, saguaro height, saguaro size, saguaro age (# of arms), surrounding vegetation, surrounding number of saguaros, proximity to buildings, ground substrate, etc.

Choose at least two variables to measure in addition to hole orientation and height. Then consider your null hypothesis to test whether woodpeckers are choosing these particular characteristics. For instance, if woodpeckers are choosing to orient their holes in a particular direction, you will see significant deviation from a random pattern whether 25% of holes point south, 25% point north, 25% point east, and 25% point west. After your discussion, make a data sheet to record characteristics of woodpecker holes which may be involved in habitat choice.

Each group should depart with the following supplies:

-measuring tape

-compass

-homemade “clinometer”

-data sheet

-map of saguaros

Locate at least 10 saguaros. Record data on all of the woodpeckers in each saguaro, but also keep track of whether more than one hole is present in a given saguaro. You want to locate as many woodpecker holes as possible, so you can also look in palm trees – be sure to record whether a hole is in a palm tree versus a saguaro.

First record the orientation of a nest hole, by standing under the hole and orient your compass to measure the direction the hole faces.

Second, you can take several measurements to use with basic trigonometry to determine the height of the saguaro or woodpecker hole. Stand some distance from the saguaro, and hold your home-made clinometer up to your eye. Find the top of what you are measuring (the saguaro or the woodpecker hole itself) through the “sight” (i.e., the straw). Determine the angle above the ground of the object you are measuring, by looking at the position of the string hanging from the clinometer: the angle is can be read with reference to the zero:

[pic]

With the measuring tape, measure the distance between the person with the clinometer and the base of the saguaro. Then measure the height of the person taking the data.

Finally, take any extra data on the saguaros that your group thought might be important in woodpecker habitat choice.

By 2:15, converge back at Koffler. Use the remaining time to calculate nest heights and nest orientations.

Using the data you collected in the field, you can calculate the height of an object as follows (obviously, the tree is a cactus in our case):

As a simple approximation, the height of the saguaro can be calculated as:

tan θ = opposite / adjacent = (height saguaro) / (distance between you and saguaro)

thus, from what you measured: height of saguaro = (distance to saguaro) * tan θ

For a more precise measure, you can take into account the height of the person measuring the angle (theta). Using this height, the same angle theta, and the equation above, you can calculate the true length of the base of your triangle. The more precise equation for the height of the cactus is thus:

Height of saguaro = [(distance from person to saguaro) + (height of person/tan θ)] * tan θ

[pic]

“Circular statistics” are normally used to analyze orientation data. But such statistics are really not fun, so we will approximate the analysis with a chi-square test. First, go through your measures of orientation and “round” the orientation to the nearest cardinal direction. Thus,

316 – 45 degrees = north

46 – 135 degrees = east

136 – 225 degrees = south

226 – 315 degree = west

Then tally your counts of hole orientations in the following table. Given your total sample size, in the second column, write what you would have expected if woodpecker chose hole direction by chance. For instance, if you counted 16 holes, you would put, “4” in each of these spaces.

| |Your Observations (total #) |Null Expectations (total #) |

|North | | |

|East | | |

|South | | |

|West | | |

|Total (N) | | |

For your data analysis, you can then type these values directly into a chi-square table online and it will say whether these distributions are statistically different. If P < 0.05, you can reject the null hypothesis that woodpeckers are choosing nest hole orientation at random, and are possibly biased their hole choice in a particular direction.

To perform this test, go to



and choose a “4” by “2” table and then enter your data directly from your table, and press “calculate chi square.”

For more power, repeat the analysis with data from the entire group.

In your lab report, include tests against your null hypothesis for nest hole orientation (chi-square). Include data on your group and the entire lab’s pooled data. Also include descriptive statistics for at least one other variable you measured (for instance, mean and variance in a graph).

References

Ardia, R.D., Perez, J.H., Clotfelter, E.D. 2006. Nest box orientation effects internal temperature and nest site selection by tree swallows. J.Field Ornithol. 77:339-344

HOWE, S., D. L. KILGORE, JR., AND C. COLBY. 1987. Respiratory gas concentrations and temperatures within nest cavities of the Northern Flicker (Colaptes auratus). Canadian Journal of Zoology 65:1541-1547.

Zwartjes, P.W., Nordell, S.E. 1998. Patterns of cavity-entrance orientation by Gilded Flickers in cardon cactus. Auk 115:119-126

Moore, W. S. 1995. Northern Flicker (Colaptes auratus). In The Birds of North America, No. 166 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.

Edwards, H. H., and G. D. Schnell. 2000. Gila Woodpecker (Melanerpes uropygialis). In The Birds of North America, No. 532 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

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Diagram shows clinometer with reading of about 10 degrees

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