TREE PREFERENCES OF FORAGING MIGRANT BIRDS
INSTRUCTIONS –– Spring ‘01
This study seeks to identify trees used for foraging by Chicago’s migrating birds in spring. The study will
consist of twenty or more pairs or groups of volunteers this spring – which will provide powerful data. A
separate study of shrubs used by migrants is also being developed. Data from the study this year will be
analyzed and results will be provided to public agencies so that they can plant tree species that are most
useful to migrants. The City of Chicago has already indicated that it intends to make use of these results.
Two or more observers working as a team will collect data in either a landscaped or a wild area
(definitions of the two types of areas are provided below). Each team will follow a transect, noting the
types of birds foraging in each tree. The transect can be a path or any route past the trees and shrubs being
sampled. Across the region, transects will be chosen to include as many of the trees that occur here as
possible.
WHEN TO OBSERVE:
You are encouraged to make as many visits to your study site as you can – the more data we get, the
better our results will be. Please make a minimum of three visits between April 15 and May 31: at least
one in April, and two in May. (You can make extra visits during that time, and before April 15 as well.)
We encourage observations early in the morning when birds are most active, but data from any time of
day will be valuable. Once you have made a map of the trees on your site, any study participant who can
identify birds can collect data alone, or you can continue to go as a pair or group.
WHERE TO OBSERVE
Study areas can be chosen in any wooded area in the Chicago Wilderness region that is frequently used by
migrating birds.
Choose either a landscaped area (park, yard, campus, forest preserve picnic area, etc.) or wild area (forest
preserve natural area or the like). The two types of area require different methods.
DETERMINING THE TRANSECT ROUTE
Before collecting any data, decide on a transect route. Here are some considerations for laying out your
route:
 Identify an area that you know is very frequently used by migratory birds during spring.
 Choose a route through this area that passes many trees of several species. The route can twist and
turn as much as needed to allow for good viewing of the different trees along it.
 Don’t make the route so long that you cannot complete it at least one time on each visit. You will
need enough time to examine each tree on the route carefully for birds once migration begins.
 Consider planning your route so that the morning sun will be behind you.
 Once your spot is chosen, classify it into one of these two groups:
 A Wild Area can be a forest preserve, or a spot in a park that looks like a natural area - for
example, large trees whose canopies overlap and branches often intermingle, with some
understory. In a wild area, choose a path or easily repeatable route. Your route should be at least
1
100 meters long. You will need to record the beginning and ending of your route on a map, so
choose easily identifiable landmarks.
 A Landscaped area is one that looks park-like and has trees that are for the most part separate and
mostly surrounded by lawn or other landscaping. When you plan your route through your
landscaped area, avoid places that could be confused with wild areas such as large intertwined
clumps of a dozen or more trees, or smaller clumps that run into the edges of natural areas. The
route should contain at least 50 trees.
GETTING STARTED
When you have an idea about where you want to establish your transect, please call Steve Frankel at
(847) 965-1150 to make sure that no one else has already claimed that area. Agree on a name for your
transect such as Harms Woods #1. It would be very helpful if you had an idea of which tree species are
on your route when you call.
On your first visit to the site, lay out your route and complete the Tree List (see method below). Choose
prominent permanent features at or near your starting or ending points so that others may find your
transect in the future. Make a map of your transect route, with the number of each tree (from your Tree
List) along it as described below, and attach it to the Tree List. This map should clearly identify the
permanent starting and ending points, have a sketch of the route, note several landmarks along the way,
and note the location of each tree. The map can be approximate – as long as it allows you and future
researchers to find and identify the trees on your route.
If you have time on your first visit, you may also record the birds you observe along your transect by
following the methods described below.
You will want to bring the map and Tree List with you on each field visit, and you may also want to
make a copy for safekeeping. One of the partners can keep this information and blank data sheets on a
clipboard to bring into the field each time. It is a good idea to make several copies of the map and tree list
so that you can keep the original in a safe, dry, cozy environment while you make your subsequent visits.
The directions that follow are in two sections. If your transect is in a landscaped
area, read the next two pages and then skip to page 8. If your transect is in a wild
area, skip the next two pages and go directly to page 5.
2
Landscaped Areas
In Landscaped Areas
DATA COLLECTION METHODS – INITIAL VISIT—TREE LIST
Your record of the tree species occurring along your route will allow us to analyze whether migrating
birds are using certain species preferentially. We not only need information on the trees that are
“popular” with migratory birds, but also trees which are particularly “unpopular” so that we can rank trees
based on their usefulness to foraging migratory birds.
Once you have established the transect route for the study, sample and map the trees along it. As you
walk along your route, identify the species of each tree you pass that is more than 4 inches (10 cm)
diameter at breast height and assign it a number. Enter its species and number on the Tree List, and
number it on your map. Use the notes section to further identify trees, for example, " broken top," "first
tree after trail," "triple trunk." One goal in making this map is to allow any team member who cannot
identify trees to use it to collect data alone. Record the diameter at breast height (dbh) of each tree, using
the method described below. You only need to collect this data once.
Measuring dbh
The dbh is simply the diameter (NOT the circumference) of the tree. Imagine a
dbh
line running straight through the trunk parallel to the ground and you’ve got it.
It is recorded approximately at chest height (about 1.5 meters) and can be
estimated once you get the hang of what a 10cm tree, a 20 cm tree, a 50 cm
tree, – or a 5 inch tree, a 10 inch tree, - etc. looks like. If the tree forks at or
1.5 m
below 1 meter from the ground, each stem should be measured as a separate
stem, but entered on the same line on the Tree List (in other words, dbh should
be taken for both stems)(see sample). On the other hand, if the tree branches
above 1 meter from the ground, then only the largest trunk is counted. If the
tree has irregularities such as a large bump, a branch, or other obstructions at chest height, then move just
below or just above that inconsistency to get the best measurement possible of the actual size of the tree.
Dbh tapes (which record dbh when they are wrapped around the tree) are available to borrow from the
Audubon Office, if you like.
 If you can’t identify a tree on your transect, you have several options:
- Wait. As the season progresses and trees flower and leaf out, you may be able to identify it.
- We may be able to send someone to walk the site with you – give us a call.
- Clip off a sample consisting of a few leaves and a section of stem including a bud. Make a
Xerox copy of it, or press it between two pieces of cardboard and send it to Rickie White,
Science Coordinator, The National Audubon Society of the Chicago Region 5225 Old
Orchard Road, Suite 37 Skokie, IL 60077.
 Disregard any tree under 4 inches (10 cm) dbh. Count any woody plant over 4 inches, even if it is
often regarded as a shrub (eg buckthorn).
 Use inches or centimeters, whichever you are most comfortable with, but be sure to note which units
you are using.
3
Landscaped Areas
 Carefully estimating the dbh is fine - if you are an inch or two off, it won’t affect the quality of our
conclusions. If you want to estimate, check yourself a few times first with a ruler or measuring tape
to be sure your estimates are reasonably accurate. (If they aren’t, you can measure your handspan,
use your 8 1/2x11 datasheet, or bring a ruler or measuring tape each time.)
 If you cannot identify the exact species of tree, please enter it as “ash sp.”, “hawthorn sp.,” etc.
 Ornamentals and special varieties of tree can be especially tricky to identify. If you run into an
unusual ornamental tree, please consider collecting a sample for identification.
 Include dead trees in your study. Identify the species if you can, and note that the tree is dead in the
“notes” column of the tree list. **
DATA COLLECTION METHODS – EACH VISIT
Complete the Environmental Conditions and the Record of Birds Observed Along the Transect for
each of your field visits.
Environmental Conditions
On the form titled Environmental Conditions, record the time that you begin and end your transect and
the wind direction when you begin, and note any remarkable weather conditions in the space provided.
Remarkable weather conditions include temperature below 45 degrees, heavy wind (small trees begin to
sway), and heavy or unseasonable precipitation. We suggest that the tree monitor record the data about
birds and vegetation, as the bird monitor may be busy with binoculars.
We encourage you to record the phenological condition (buds only [no leaves or flowers], flowering,
leaves small and emerging, leaves mature) of one typical example of each tree species as you walk your
route, although it is not absolutely necessary that each team does this. You can do this at the time you
collect bird data, or on a second walk along the transect. Listing the different tree species in your transect
on the Environmental Conditions form and Xeroxing enough copies for each visit will simplify this.
Record of Birds Observed Along the Transect
The sampling unit we have chosen to use in this study is the tree, not the bird. The method of data
collection is to walk from tree to tree, observing the birds in each. Each foraging choice that a bird makes
may influence its subsequent choices, so that using a bird as the sampling unit and recording its foraging
choices as it moves from tree to tree would introduce an unwanted complication into the data analysis.
Therefore, don’t follow birds, follow trees .
Walk the pre-determined transect route, visiting each tree in order. Spend enough time to carefully check
each tree for birds. When you see a migrant foraging, list the name of the bird and the tree in which it is
foraging in the appropriate place on the Record of Birds Observed Along the Transect. List all the
birds that are in that tree. Then move on to the next tree. Only record data for trees that are on your Tree
List from your initial visit.
Skip ahead to page 8
4
Wild areas
In Wild Areas
DATA COLLECTION METHODS – INITIAL VISIT—TREE LIST
Your record of the tree species occurring along your route will allow us to analyze whether migrating
birds are using certain species preferentially. We not only need information on the trees that are
“popular” with migratory birds, but also trees which are particularly “unpopular” so that we can rank trees
based on their usefulness to foraging migratory birds.
Once the route for the study has been established, sample and map the trees along it. As you walk along
your route, identify each tree within 5 meters of the trail on either side that is more than 4 inches (10 cm)
diameter at breast height, and assign it a number. Enter its species and number on the Tree List and
number it on your map. Use the notes section to further identify trees, for example, " broken top," "first
tree after trail," "triple trunk."
Record the diameter at breast height of each tree, using the method described below. A suggested method
is for the tree monitor to walk through the woods, keeping 5 meters from the trail, and call off the names
and dbh’s of all trees between him or her and the trail. The bird monitor can walk the trail with the
clipboard, recording the information.
If the vegetation is so dense that you will not be able to see birds in trees that are less than 5 meters from
the trail, then only record dbh on the trees that are visible from the trail in that section. In wild areas, you
may need to have a fairly long route to increase your likelihood of seeing birds. If this is the case, you
may subsample the route using the method described below.
Sub-sampling method **
If your transect is long, you will need to subsample. Instead of taking tree data on a 5 meter band on both
sides of the trail, record one side only. If your transect is very long, you can reduce the width of the
sampling band to 3 or 4 meters. The subsampling applies only to the trees. Take data for birds along
your entire route, whether you have subsampled or not.
Measuring dbh
The dbh is simply the diameter (NOT the circumference) of the tree. Imagine a
line running straight through the trunk parallel to the ground and you’ve got it.
It is recorded approximately at chest height (about 1.5 meters) and can be
estimated once you get the hang of what a 10cm tree, a 20 cm tree, a 50 cm
tree, – or a 5 inch tree, a 10 inch tree, - etc. looks like. If the tree forks at or
below 1 meter from the ground, each stem should be measured as a separate
stem, but entered on the same line on the Tree List (in other words, dbh should
be taken for both stems)(see sample). On the other hand, if the tree branches
5
dbh
1.5 m
Wild areas
above 1 meter from the ground, then only the largest trunk is counted. If the tree has irregularities such as
a large bump, a branch, or other obstructions at chest height, then move just below or just above that
inconsistency to get the best measurement possible of the actual size of the tree.
Dbh tapes (which record dbh when they are wrapped around the tree) are available to borrow from the
Audubon Office, if you like.
 If you can’t identify a tree on your transect, you have several options:
- Wait. As the season progresses and trees flower and leaf out, you may be able to identify it.
- We may be able to send someone to walk the site with you – give us a call.
- Clip off a sample consisting of a few leaves and a section of stem including a bud. Make a
Xerox copy of it, or press it between two pieces of cardboard and send it to Rickie White,
Science Coordinator, The National Audubon Society of the Chicago Region 5225 Old
Orchard Road, Suite 37 Skokie, IL 60077.
 Disregard any tree under 4 inches (10 cm) dbh. Count any woody plant over 4 inches, even if it is
often regarded as a shrub (eg buckthorn).
 Use inches or centimeters, whichever you are most comfortable with, but be sure to note which units
you are using.
 Carefully estimating the dbh is fine - if you are an inch or two off, it won’t affect the quality of our
conclusions. If you want to estimate, check yourself a few times first with a ruler or measuring tape
to be sure your estimates are reasonably accurate. (If they aren’t, you can measure your handspan,
use your 8 1/2x11 datasheet, or bring a ruler or measuring tape each time.)
 If you cannot identify the exact species of tree, please enter it as “ash sp.”, “hawthorn sp.,” etc.
 Ornamentals and special varieties of tree can be especially tricky to identify. If you run into an
unusual ornamental tree, please consider collecting a sample for identification.
 Include dead trees in your study. Identify the species if you can, and note that the tree is dead in the
“notes” column of the tree list. **
DATA COLLECTION METHODS – EACH VISIT
Complete the Environmental Conditions and the Record of Birds Observed Along the Transect for
each of your field visits.
Environmental Conditions
On the form titled Environmental Conditions, record the time that you begin and end your transect and
the wind direction when you begin, and note any remarkable weather conditions in the space provided.
Remarkable weather conditions include temperature below 45 degrees, heavy wind (small trees begin to
sway), and heavy or unseasonable precipitation. We suggest that the tree monitor record the data about
birds and vegetation, as the bird monitor may be busy with binoculars.
We encourage you to record the phenological condition (buds only [no leaves or flowers], flowering,
leaves small and emerging, leaves mature) of one typical example of each tree species as you walk your
route, although it is not absolutely necessary that each team does this. You can do this at the time you
collect bird data, or on a second walk along the transect. Listing the different tree species in your transect
on the Environmental Conditions form and Xeroxing enough copies for each visit will simplify this.
6
Wild areas
Record of Birds Observed Along the Transect
The sampling unit we have chosen to use in this study is the tree, not the bird. The method of data
collection is to walk from tree to tree, observing the birds in each. Each foraging choice that a bird makes
may influence its subsequent choices, so that using a bird as the sampling unit and recording its foraging
choices as it moves from tree to tree would introduce an unwanted complication into the data analysis.
Therefore, don’t follow birds, follow trees .
Walk the transect route, checking the trees on both sides of the trail for birds. Stop when you come to a
tree with a foraging bird or birds, note the tree species, and list names of the migrant birds you see
foraging in that tree in the appropriate place on the Record of Birds Observed Along the Transect. You
do not need to restrict yourself to trees within 5 meters of the trail. Record any sightings if you can clearly
identify the foraging bird species and the tree it is in from the transect route. If the bird is in a tree that is
not on your Tree List, check the appropriate column. Don’t worry if you subsampled the trees. Walk
your entire route and record any bird you see clearly – just as the instructions above describe.
7
Some important points:











Use the accompanying list to determine which migrant bird species to report in this study. You can
record additional bird species if you like, but that is optional and the data will be treated separately.
Please do not abbreviate species names on the forms that you mail in.
Count only foraging birds: any bird moving around from branch to branch is most likely foraging and
should be counted - unless it is clearly not foraging (being chased, for example). Do not count
stationary birds.
If you cannot identify a bird but can identify the family, please include it – for example, warbler sp.,
thrush sp., etc.
Identify as many birds as you can that are foraging in the tree, and then move on to the next one. Try
not to spend more than 5 minutes at a particular tree.
If a bird that you have already counted has flown into another tree on your route and is there when
you approach the tree, record it again.
If you see two or more of the same bird species in one tree, record the number with crosshatches next
to the name of that species.
If you hear a bird singing, try to detect what tree it is in as you move along your transect. Record it
only if you observe that it is foraging (using the standard above).
If you see very few birds as you walk your transect, consider walking it again. Use a new form for
the second walk. You may walk the transect as often as you like, as long as you are careful to
examine each tree in order.
If you have a theory that trees near water are more used, or oaks that are near cherries, or trees that
get morning sun, etc., we encourage you to collect additional data for your own use and let us know
about your results. If your data suggests that we're onto something important, we may be persuaded to
pursue that through this study in a more organized way in future years.
Thank you heartily in advance for resisting the temptation to follow a flock of birds around or rush
ahead to a tree with birds. The reliability of this study depends on the care you take in following
these instructions.
WHAT TO DO WITH THE DATA
By June 20, please return all data sheets to:
Steve Frankel, Field Representative
The National Audubon Society of the Chicago Region
5225 Old Orchard Road, Suite 37
Skokie, IL 60077.
If you prefer to return the data by e-mail, and would like an excel spreadsheet template to record your
data on, contact rwhite@audubon.org.
If you have any questions, contact Judy Pollock or Rickie White at (847) 965-1150 Or
jpollock@audubon.org, rwhite@audubon.org.
** We have asterisked items that we are unsure of and particularly interested in feed back from our
reviewers about.
8