Owl Monitoring, ME - National Audubon Society

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Owl Monitoring, ME - National Audubon Society

Conservation Status and Volunteer Monitoring of Maine Owl Populations

Project # 002-01-06

Final Report to the Maine Outdoor Heritage Fund

_______________________________________________________

Thomas P. Hodgman,

Wildlife Biologist, Bird Group,

Maine Dept. of Inland Fisheries and Wildlife,

650 State St., Bangor, ME 04401

And

Susan M. Gallo,

Wildlife Biologist, Maine Audubon,

20 Gilsland Farm Road, Falmouth, ME 04105

26 February 2004


Introduction

Owls are an integral part of America’s outdoor heritage and especially important in

children’s literature and popular culture. Yet, the nocturnal habits and winter breeding

season of owls make them one of the most poorly understood groups of birds in North

America. They are thought to be sensitive to the effects of human disturbance and forest

fragmentation because of their large area requirements. As with other charismatic

species, like puffins, moose, and loons, the public is extremely interested in learning

more about owls. This is especially true in Maine where the average citizen has greater

contact with and knowledge of the outdoors than in most other eastern states. Owls are

extremely popular with the general public, and consequently, volunteers have been used

to assist with data collection (Duncan and Duncan 1997, Francis and Whittam 2000).

Owls are often used to symbolize wisdom and knowledge. In stark contrast, however, is

our own lack of knowledge regarding owls. Although six species are believed to breed in

Maine, only three species appear common (Adamus 1987). At the outset of our project,

we had only limited data on species distributions (Adamus 1987) and no reliable data on

their population status and trends in Maine. Current monitoring programs for nongame

birds, like the Breeding Bird Survey and Christmas Bird Count, largely miss owls

because they do not take place during the time of year or time of day when owls are most

vocal and easily detected. Without such information, biologists have no data upon which

to base land management recommendations that will ensure long-term persistence of owl

populations in Maine. This lack of information is compounded by disagreements in the

scientific literature as to the best methods to assess and monitor this array of species.

Innovative strategies are needed to help agencies respond to an ever-increasing public

demand for nongame conservation. Agencies and nonprofit organizations alike are

increasingly using volunteers to meet this demand. Aside from the obvious benefits to

conservation (i.e., expanded data collection at reduced cost), volunteers gain personal

satisfaction and empowerment when they contribute to broad-scale conservation

programs. The objectives of this project were four-fold: to collect data on abundance and

distribution of owls throughout a large part of Maine, to evaluate potential survey

methods and the influence of environmental conditions, to evaluate the effectiveness of

monitoring, and to build a network of volunteers capable of carrying out these objectives

and a future long-term monitoring program.

Route-based Surveys

Methods

We created 190 roadside survey routes using the DeLorme atlas to facilitate layout. We

divided each page of the atlas into quadrants and assigned one route per quadrant. When

volunteer interest exceeded availability of routes, we placed two routes per quadrant, but

without overlapping them. Route layout was not statewide, lacking primarily in the

northwest portion of Maine.

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We used volunteer observers to conduct three roadside surveys for owls each year. We

trained volunteers to recognize the calls of most Maine owls and provided them with a

cassette tape or compact disc to use during surveys. Each volunteer (or group of

volunteers) was assigned a 10-12 mile route along which they were required to stop at

eight locations separated by >1 mile. At each stop, volunteers recorded each individual

owl heard calling and the minutes in which that individual was heard over a 16-minute

survey period. Volunteers began each stop with three minutes of “passive” (i.e., no

playback) listening followed by a 20-second playback sequence (first species), three

minutes listening, a 20-second playback sequence (second species), six minutes listening,

a 20-second playback (third species), and three minutes listening.

Detectability

Surveys were scheduled so that each volunteer surveyed in the beginning, middle, and

end of the field season (late January through late April). Each survey also was randomly

assigned a time period (early: 1900-2200 h, middle: 2200-0100 h, late: 0100-0400) so

that each volunteer would conduct one survey in each time period. This allowed us to

examine the effects of both seasonal and nightly timing on detectability, specifically to

identify when a single survey would produce the maximum number of detections.

Volunteers measured other factors potentially influencing detectability of owls during

roadside surveys, including cloud cover, precipitation, wind speed, temperature and the

number of cars that passed during the survey.

We also investigated the efficacy of broadcasting owl calls for inducing a response. We

used a three-minute passive listening period at the beginning of each stop as a control to

gauge the effectiveness of playbacks. We broadcast the vocalizations of Northern Sawwhet

Owl, Barred Owl, and Great Horned Owl. A listening period, as described above,

followed each 20-second broadcast. Furthermore, we alternated the first and last species

broadcast between Northern Saw-whet Owl and Great Horned Owl to determine if the

smaller owl would be intimidated by calls of the larger species. Barred Owl was always

the second species in the broadcast sequence.

Survey Effort

Results

During our two-year study, we conducted 608 individual surveys on 146 individual

routes. An additional 44 routes were assigned, but no surveys were conducted (no data

were submitted from them). In 2002, we enlisted 260 volunteers for 166 routes and 192

volunteers for 138 routes in 2003. The actual number of routes surveyed at least once

was 127 in 2002 and 92 in 2003. Participation rate (number of routes submitting

data/number of routes assigned) decreased from 77% in 2002 to 67% in 2003. Mean

number of surveys per week was 22.3 in 2002 and 15.5 in 2003. Survey effort by time of

night was relatively constant, for both years, despite assigning each volunteer three

different time periods (Fig. 1).

3


Distribution and Abundance

Our data indicated Northern Saw-whet Owls, Barred Owls, and Great Horned Owls were

distributed statewide as expected. Volunteers also recorded several detections of rare

species including 5 Eastern Screech-owls, 11 Long-eared Owls, and 3 Short-eared Owls.

No Boreal Owls or Common Barn Owls were detected despite training volunteers to

recognize their vocalizations. Too few detections of Long-eared Owls were recorded to

draw conclusions regarding their distribution in Maine. Short-eared Owls are most likely

still “wintering” during all but the last few weeks of the study period so no conclusions

regarding their breeding status can be reliably drawn. Based on our limited data and

anecdotal reports given to us as a result of the publicity surrounding the project, Eastern

Screech-owls appeared to occur annually, though sparsely, throughout southern and

central Maine.

We recorded 1457 detections of individual owls. Barred Owls were the most abundant

(670 individuals detected), followed by Northern Saw-whet Owls (502), with Great

Horned Owls contributing the least number of detections (266). Half of the survey routes

(73) were run in both years with 92% (67) of routes reporting at least one owl in both

years. The remaining 73 routes were run just in one year, and on only six of these were

no owls detected.

Detectability

Environmental conditions had little overall effect on detectability. Neither moon phase

nor cloud cover influenced the number of owls heard on surveys (Figs. 2, 3, 4).

Temperatures below 10 o F appeared to be associated with a reduction in detections (Fig.

5). However, we asked volunteers specifically not to conduct surveys below 0 o F, so

admittedly, our data were limited at cold temperatures.

Seasonal timing clearly affected the number of owls heard along our roadside routes.

Between-year variation also may influence probability of detection. In general, numbers

of Northern Saw-whet Owl calls heard increased from a low in late January (both years)

to a high in mid-March (Fig. 6). In both years, numbers of Northern Saw-whet Owl calls

began to decline from early to mid-April. Barred Owls, however, displayed an opposite

pattern with a generally slow steady increase in the number of calls heard per route from

late January through late April in both years (Fig. 7). Great Horned Owls exhibited a

bimodal response with a first peak in late February/early March followed by a second

increase in detections in early April (Fig. 8).

The time of night when surveys were conducted was an important influence on the

number of detections recorded. For all three species, the number of detections was

greater in the late (0100-0400 h) time period (Fig. 9). This relationship was most striking

for Barred Owls (Fig. 10) and Great Horned Owls (Fig. 11) and slightly less so for

Northern Saw-whet Owls (Fig. 12). Peak period for the latter species may occur slightly

earlier in the evening than for other species. Although some annual variation existed in

4


these data, the overall relationships are consistent with peak numbers of detections

occurring after midnight.

Owl response to broadcasts varied greatly by species. Northern Saw-whet Owls

exhibited a strong positive response to playback of conspecific vocalizations (Fig. 13).

The number of individuals responding increased approximately two-fold over the initial

passive listening period and nearly three-fold when their vocalization was broadcast as

the third species in the sequence. Barred Owls also increased their vocalizations 1.5- to

two-fold over the previous 3-minute listening period (Fig. 13). Great Horned Owl

detections increased little and only in the final three minutes suggesting that playback

order somehow may affect their response (Fig. 13).

The order in which species are broadcast influenced the number of owls responding.

This was most striking for Northern Saw-whet Owls, but evident in other species as well

(Fig. 14). Total detections of Northern Saw-whet Owls were greater when their

vocalization was broadcast first than when it was the third species broadcast. Barred Owl

detections should not differ because they were always the second species broadcast.

However, more were detected when the Northern Saw-whet was the first species in the

sequence (Fig. 15). Great Horned Owl detections increased markedly when we broadcast

their vocalization third (i.e., Northern Saw-whet Owl first) (Fig. 16). Interestingly, Great

Horned Owls also appeared to increase their responses about three to five minutes after

broadcast of the Barred Owl vocalization.

Field Surveys

Discussion

This project was the first attempt since Maine’s Breeding Bird Atlas Project (Adamus

1987) to understand the status of these nocturnal species in Maine. Unfortunately, the

entire state was not equally covered by our surveys. Significant gaps in western,

northern, and eastern Maine existed. Essentially our coverage was adequate wherever

people and plowed roads were in abundance. Consequently, our conclusions regarding

abundance and distribution only apply to those areas “adequately” surveyed. With

careful planning and permission from large forest landowners surveys could be

conducted in the unorganized townships, provided roads that are plowed every year can

be identified. Habitat along these main haul roads, however, may not be representative of

the area overall.

Evident in our data was a spike in the number of detections for the first minute of the

passive listening period (Figs. 14, 15, 16). We believe these data are suspect and not a

good indication of the number of owls calling on their own. Instead, we believe that

volunteers may have started their passive listening period upon first hearing an owl call.

Specifically, while getting organized and waiting for the beginning of the time period,

volunteers may have initiated their survey a minute or two early because they heard an

owl call and did not want to exclude it from the data, believing that it might not call

5


again. This spike in detections for the first minute is consistent across all species and

regardless of order of species on the playback. Consequently, our comparison of passive

versus playback detections is weakened. We believe that if the data for passive

detections were recorded correctly, the differences in detectability between passive

listening and using playbacks would be much greater.

Our data provide clear evidence that regardless of year and species, the highest

probability of detection occurs after midnight. However, this may not be solely a

biological function. During the early and middle time periods (i.e., 1900-2200 h, 2200-

0100 h), we recorded more passing vehicles and other disturbances such as dogs barking,

snowmobiles, etc. It is possible that noise interference alone could reduce the volunteer’s

ability to hear and identify calling owls, thus reducing the number of detections heard

before midnight.

Our surveys were conducted in a three-month period when all three of the common owl

species would be most vocal. It was an overall goal to identify a narrow window (3-4

week period) when all species could be detected with minimal survey effort. The single

most effective period probably occurs sometime from mid March to early April (Figs. 6,

7, 8). Surveys earlier in the season, however, may be necessary to best document

occurrences of Great Horned Owl. It appears from Figure 8 that this period may vary

from year to year, where in 2002 a clear spike in detections occurs during the first week

of March. In 2003, however, no such sharp peak occurs; rather a smaller peak one week

earlier followed by general increase from mid March through mid April. During the last

week of surveys (late April), we observed a final increase in detections. Years of high

prey abundance positively influences clutch size and contributes to earlier egg laying in

Great Horned Owls (Houston 1971). If so, then perhaps differences in prey availability

influenced the annual variation in periods of peak calling that we observed. For Barred

Owls, frequency of detections increased throughout the survey. Although Barred Owls

were easily detected and the most abundant species documented on the survey, we did

not identify the peak of calling for this species (Fig. 7). Northern Saw-whet Owls are

generally thought to be migratory, though this is perhaps not the case throughout the

entire state, as some detections occurred in January ahead of any expected spring

migration. The sharp peak of detections heard in mid March 2003 and a smaller, broader

increase from late February through mid April 2002 probably represent a pulse of

migrants for that year (Fig. 6). Data from our survey, until approximately mid March,

may represent largely migrants eager to respond to playbacks. Palmer (1949) reported

egg dates for Northern Saw-whet Owls as early as the first week of April for northern

Oxford County.

The conclusion of our project takes place as other states have expressed interest in

starting similar projects and an initiative known as Coordinated Bird Monitoring (CBM)

is starting to take hold nationwide. As a result of this outside interest, our project has

been identified as one that might be used as a model for other states. We will continue to

conduct owl surveys for at least the next few years using a protocol modified slightly

from that described in Methods above. Significant changes include:

6


One survey per year starting early March and concluding early April.

Surveys begin at midnight and conclude by 0400 hours.

Number of stops per route increased from eight to ten.

Use playbacks in the following order: Northern Saw-whet Owl, Barred Owl,

Great Horned Owl.

Listening period following first and third playback reduced to two minutes.

Conduct surveys only when overnight minimum temperatures are >10 o F.

A complete summary of all methods can be found in Appendix A. Details regarding

changes to protocol can be found in Appendix B. Both appendices are provided to

volunteers via the Maine Audubon website.

Power Analysis

When we proposed our project to sponsors, we intended to conduct a power analysis to

determine the potential ability of our monitoring efforts to detect population trend for this

group of birds in Maine. At that time, we planned to use the program MONITOR which

was available for free download. That program is no longer in use and has been removed

from the USGS website. Apparently, some limitations of this software have been

discovered and erroneous conclusions could be drawn from its output. As a consequence,

we have not conducted a power analysis on our data. However, we have initiated

discussions with federal researchers and biologists from other states, as part of CBM to

pool portions of our data with those of others and conduct more detailed analyses aimed

at a regional, and perhaps, a national monitoring effort. It is our intention to cooperate

with this effort and our hope that the data collected during this project will be used to

develop a monitoring program at a larger geographic scale. At that point perhaps, better

guidance with regard to power analysis will be available and can be conducted on a larger

dataset than we have today.

Volunteer Database

Our project used hundreds of volunteers to complete the field work. Appendix C

provides a summary database of the volunteers participating in this project. Many of

these individuals have expressed interest in continuing with our revised monitoring

efforts for owls in Maine.

Literature Cited

Adamus, P. R. 1987. Atlas of breeding birds of Maine, 1978-1983. Maine Dept. of

Inland Fisheries and Wildlife, Augusta, ME, 366pp.

Duncan, P. A, and J. R. Duncan. 1997. Increase in distribution records of owl species in

Manitoba based on a volunteer nocturnal survey using boreal owl (Aegolius

funereus) and great gray owl (Strix nebulosa) playback. Pages 519-524 in J. R.

7


Duncan, D. H. Johnson, and T. H. Nichols, eds. Biology and conservation of owls

of the northern hemisphere. USDA Forest Service General Technical Report NC-

190.

Francis, C. M. and B. Whittam. 2000. Ontario Nocturnal owl survey: 1999 pilot study

final report. Ontario Ministry of Natural Resources. 34pp.

Houston, C. S. 1971. Brood size of the great horned owl in Saskatchewan. Bird-Banding

42:103-105.

Palmer, R. S. 1949. Maine Birds. Bulletin of the Museum of Comparative Zoology.

Volume 102. Harvard College, Cambridge, Massachusetts. 656pp.

8


Figure 1. Survey Effort by Time of Night

# of Surveys

140

120

100

80

60

40

20

0

1900-2200 Hours 2200-0100 Hours 0100-0400 Hours

Survey Time

2002

2003

# Owl Detections/Survey

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Figure 2. Number of Detections/Survey by Moon

Phase

BARR

GHOW

NSWO

0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-100

New % Lunar Cycle Full

9


Figure 3. Number of Detections/Stop by Cloud Cover

0.18

0.16

0.14

BARR

GHOW

NSWO

# Owls/Stop

0.12

0.10

0.08

0.06

0.04

0.02

0.00

0 10 20 30 40 50 60 70 80 90 100

% Cloud Cover

Figure 4. Number of Detections/Survey by

Moon Visibility

# Detections/Survey

2.00

1.80

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

BARR

GHOW

NSWO

0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-

100

% Visibility (Moon and Clouds)

10


# Owls Detected/Survey

Figure 5. Number of Owls

Detected/Survey by Temperature

4

3.5

3

2.5

2

1.5

1

0.5

0

-25 to

-21

-20 to

-16

-15 to

-11

-10 to

-6

-5 to

-1

Degrees Celsius

0-4 5-9 10-14 15-19

Figure 6. Number of Detections/Route by

Week and Year

# Owl Detections/Route

2.50

2.00

1.50

1.00

0.50

0.00

NSWO02

NSWO03

29 Jan

5 Feb

12 Feb

19 Feb

26 Feb

5 Mar

12 Mar

19 Mar

26 Mar

2 Apr

9 Apr

16 Apr

23 Apr

Week

11


# Owl Detections/Route

2.50

2.00

1.50

1.00

0.50

0.00

Figure 7. Number of Detections/Route by

Week and Year

BARR02

BARR03

29 Jan

5 Feb

12 Feb

19 Feb

26 Feb

5 Mar

12 Mar

19 Mar

26 Mar

2 Apr

9 Apr

16 Apr

Week

23 Apr

# Owl Detections/Route

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Figure 8. Number of Detections/Route by

Week and Year

29 Jan

5 Feb

12 Feb

19 Feb

26 Feb

5 Mar

12 Mar

19 Mar

26 Mar

2 Apr

9 Apr

16 Apr

Week

GHOW02

GHOW03

23 Apr

12


0.25

Figure 9. Owls Detected/Stop by Hour (Both Years Combined)

# Owls Detected/Stop

0.2

0.15

0.1

0.05

0

6:00:00 PM

7:00:00 PM

8:00:00 PM

9:00:00 PM

10:00:00 PM

11:00:00 PM

12:00:00 AM

1:00:00 AM

Hour

2:00:00 AM

3:00:00 AM

4:00:00 AM

5:00:00 AM

BARR

GHOW

NSWO

# Owls Detected/Stop

Figure 10. Barred Owl Detections per Stop

by Hour and Year

0.300

0.250

0.200

0.150

0.100

0.050

0.000

6:00:00 PM

7:00:00 PM

8:00:00 PM

9:00:00 PM

10:00:00 PM

11:00:00 PM

12:00:00 AM

1:00:00 AM

Hour

2:00:00 AM

3:00:00 AM

4:00:00 AM

5:00:00 AM

BARR02

BARR03

13


# Owls Detected/Stop

0.300

0.250

0.200

0.150

0.100

0.050

0.000

Figure 11. Great Horned Owl

Detections/Stop by Hour and Year

6:00:00 PM

7:00:00 PM

8:00:00 PM

9:00:00 PM

10:00:00 PM

11:00:00 PM

12:00:00 AM

1:00:00 AM

Hour

2:00:00 AM

3:00:00 AM

4:00:00 AM

5:00:00 AM

GHOW02

GHOW03

Figure 12. Northern Saw-whet Owl

Detections/Stop by Hour and Year

# Owls Detected/Stop

0.300

0.250

0.200

0.150

0.100

0.050

0.000

6:00:00 PM

7:00:00 PM

8:00:00 PM

9:00:00 PM

10:00:00 PM

11:00:00 PM

12:00:00 AM

1:00:00 AM

Hour

2:00:00 AM

3:00:00 AM

4:00:00 AM

5:00:00 AM

NSWO02

NSWO03

14


# Responses

250

200

150

100

Figure 13. Total Number of Owl Responses for Each Minute of

Survey

BARR

GHOW

NSWO

50

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Survey Minute

Figure 14. First NSWO response by tape

(A=300 surveys, B=308 surveys)

60

50

NSWO A

NSWO B

# Owls Detected

40

30

20

10

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Survey Minute

15


Figure 15. FIRST BARR response by tape

(A=300 surveys, B=308 surveys)

# Owls Detected

40

35

30

25

20

15

10

5

0

BARR A

BARR B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Survey Minute

Figure 16. First GHOW response by tape

(A=300 surveys, B=308 surveys)

25

20

GHOW A

GHOW B

# Owls Detected

15

10

5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Survey Minute

16

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