Common Nighthawks, Chimney Swifts, and Citizen Scientists in Milwaukee
William P. Mueller, Bryan B. Lenz, and Michael U. Schlotfeldt
Abstract
The Western Great Lakes Bird & Bat Observatory (WGLBBO, recently renamed the Lake Michigan Bird Observatory) conducted a multi-pronged effort that focused on several important areas of urban bird conservation: monitoring and conservation of Chimney Swift (Chaetura pelagica) and Common Nighthawk (Chordeiles minor). These two urban-nesting species are in decline and their use of urban habitat creates an opportunity for both non-traditional urban monitoring using citizen scientists as well as novel conservation actions. This project focused on the former to facilitate follow-up research on the latter.
Introduction
We monitored urban Chimney Swift (Chaetura pelagica) and Common Nighthawk (Chordeiles minor) numbers in urban habitat in Milwaukee County, Wisconsin, during June-October of 2018. We ran 17 routes of slightly varying lengths, along streets in the City of Milwaukee and adjacent communities, with 25 participants collecting data during five-minute point counts, placed approximately six city blocks apart on the routes. We had participants note active chimneys, as used by nesting Chimney Swift pairs, swift “helpers”, and roosts. Additional data was collected and compared to other areas in Wisconsin regarding chimney flues and whether or not swifts used chimneys lined with short flues. Notes were taken on approximate height of foraging swifts.
Funding
This research was funded by a U.S. Fish & Wildlife Service grant.
Methods
We managed a group of 25 citizen scientists who collected data on Chimney Swift and Common Nighthawk numbers and movements in the City of Milwaukee and surrounding communities, all of which are in Milwaukee County. Citizen scientists conducted weekly five-minute point counts at a total of 828 points, recording birds seen or heard. Points were organized in a set of 17 approximately-linear monitoring routes (Appendix A), with each point spaced approximately six city blocks apart. This design allowed observers to drive or bike between points and still have their transportation with them upon completion of their routes, as opposed to linear routes done all on foot. The total amount of time to complete each route was approximately one hour. The first point on each transect was sampled at 45 minutes before sunset, which was calculated using NOAA’s online calculator (https://www.esrl.noaa.gov/gmd/grad/solcalc/). We rounded exact start times to the nearest five-minute interval to simplify data collection for the citizen scientists. Each transect was completed weekly - citizen scientists were asked to space transect sampling by as many days as possible, but the only restriction was that individual transects could not be sampled on consecutive days.
Participants noted approximate height above tree or building top when observing swifts: A = 0-50 feet above tree/building tops; B = 50 feet or more. Observers also noted compass bearing: (Compass headings are the two points between which the observer noted birds, and what we termed “Roost/Chimney Proximity” (whether birds were seen immediately near a nest or roost); few observers noted anything in this category.
This study focused on breeding birds, so we collected data from June 5, 2018 through October 2, 2018 to try to reduce the number of migrants in our dataset. Our citizen scientists did a varyingly-complete set of surveys – some did all of them with few misses, while others did so less successfully. Other citizen-based monitoring efforts are either currently in place or have been done within this species’ geographic range (Mordecai 2008).
The authors counted the total number of chimneys found within the observer’s view within 360 degrees around each point and recorded whether each chimney was capped or not. Our data provide several additional insights into swift ecology: (a) foraging extremely high above ground to exploit ephemeral insect prey sources, and (b) flight behavior that is incompletely described in some of the literature.
OVERALL DATA SUMMARY
Observers completed 828 total point counts along our 17 transects, each with six points. Routes were never truncated but there was variation in the number of surveys of each route due to differing participation rates among citizen scientists (Range = 2-16; mean = 8.1; median = 9; std dev = 5.2).
CHIMNEY SWIFT SUMMARY
We conducted 828 point counts (Range = 0-124; total birds = 2209; mean = 2.67, std dev = 6.703). Chimney Swifts were recorded on 398 point counts (398/828=48.07%), with a mean group size of 5.55 birds (Range = 1-124; std dev = 8.806).
At each point, observers counted the total number of visible chimneys
(Range = 0-16; Mean = 7.29; Std Dev = 3.957) and the total number of visible
capped chimneys (Range = 0-14; Mean = 4.63; Std Dev = 3.287) in an attempt to
create a proxy for neighborhood age and nest space availability. We ran a
Kruskal Wallis test to look at the influence of these two variables on the
number of Chimney Swift recorded at each point (Range = 0-124; Mean = 2.67; Std
Dev = 6.702). The test showed no relationship for capped chimneys (X2 = 32.785; df = 27; p = 0.204) and a weak but significant
relationship for total chimneys (X2
= 50.770; df = 27; p = 0.004). To
more closely examine the relationship between total chimneys and number of
Chimney Swifts, we used the data from 6/5/2018 – 8/13/2018 to run a Spearman
Rank Correlation between the number of Chimney Swifts counted at a point and
the number of chimneys visible from the corner and the created a scatter plot
with a fit line (rS =
-0.108; N = 510; p = 0.014). 
COMMON NIGHTHAWK SUMMARY
Our Common Nighthawk analyses do not parallel those for Chimney Swift because the temporal distribution of sightings shows that the individuals recorded were migrant birds. We conducted 828 point counts (Range = 0-45 birds; Total birds = 98; Mean per count = 0.12; St dev = 1.746). We detected birds on only 17 of 828 point counts (2.05%) (Range = 1-44; Mean = 5.76 birds; Std dev = 11.094).
Common Nighthawk
Noteworthy during this pilot study is the fact that out of this group of 25 observers doing regular monitoring routes across a wide swath of the urban Milwaukee area, no Common Nighthawks were observed during the breeding season (June and July). We did observe migrating nighthawks on nine dates in August and September, typical in our region, with one count recording 45 individuals. Based on these surveys and the senior author’s unpublished observations over a half century (WPM, unpublished data), we find that the Common Nighthawk has nearly ceased to be a regular breeding species in urban Milwaukee. The nighthawks that are still regularly observed during spring migration, and more commonly during autumn migration, also show a decline as they are currently seen in a fraction of former numbers (WPM, unpublished data).
Nightjar surveys can be challenging for a range of reasons (Bender and Brigham 1995), so authorities on this species recommend that care should be taken when investigating population trends. Analysis of Breeding Bird Survey (BBS) data seems to point to a general decline in Canada (50% since 1966) and in the United States measured over both long (1966-2002) and shorter time periods (1982–2002) (Environment Canada 2016, Sauer et al. 2014, Brigham et al. 2011). This is a dramatic change from past decades, when nighthawks were abundant in urban Milwaukee. 2018 breeding season records in eBird in the urban region of Southeast Wisconsin show less than six records from Milwaukee County (outside of and separate from our monitoring routes). WPM noted regular nightly Common Nighthawk sightings in summer in this identical geographic area, beginning in the late 1950s (WPM personal communication/unpublished data).
Discussion
Chimney Swift
Our results suggest that the insights and conclusions from the Ontario study on Chimney Swifts done by Fitzgerald et al. (2014), namely that they (“established a volunteer-based survey to inventory and describe chimneys [n = 928] that were used or unused by swifts. A logistic regression model showed that "swifts preferred chimneys with a greater length exposed above the roofline and greater inside area, which were not associated with residential buildings.” (Fitzgerald et al. 2014, p. 507) are dissimilar to ours. Their findings may not be repeated in Milwaukee as our analysis shows a weak negative relationship between areas with few to no chimneys and Chimney Swift presence. There are two possible explanations for this:
1. The number of chimneys visible at the point counts is a poor proxy for habitat quality/availability, which would require additional study to determine which fine-scale characteristics of chimneys make them useable as swift nest sites.
2. As the number of available nesting sites increases, Chimney Swifts are more uniformly distributed thereby reducing their numbers at individual points.
In older areas of Milwaukee chimneys are common, but in many neighborhoods a high proportion of these chimneys are capped. In these neighborhoods, remaining chimneys used by swifts (either as nesting or roosting sites) are often sited on older non-residential buildings, such as schools, small factories and businesses, parish buildings, stores, and similar commercial-industrial-educational buildings (WPM observations; this study), and are often larger in size. Residential dwellings with chimneys in those older neighborhoods may no longer be usable by swifts because so many are capped, are too small to enter (Steeves et al. 2020), and/or are too smooth on the inside due to ceramic or metal flues or chimney liners that run the length of the chimney and are not adequate substrate for swift nests. Nests were not found in chimneys with a diameter of less than 25 centimeters in the Steeves et al. study (can be expressed as a cross-sectional area of 490 square centimeters) in Kansas (Cink, in Steeves et al. 2020).
Residences in newer areas often have few to no chimneys at all (newer-style furnaces vent on the side of the building), unless associated with a fireplace.(International Residential Code 2021).
In addition to monitoring swifts and nighthawks, we assessed the number of capped vs. un-capped chimneys at each stop on all monitoring routes done this calendar year. We and other researchers are learning what makes a given chimney useful as nest substrate for swifts (leRoux and Nocera 2020). Swifts do use other sites for nesting, including air shafts on larger buildings (Dexter 1969) and other studies have found that they demonstrate a preference for larger chimneys, especially during the roost periods (leRoux and Nocera 2020, Steeves et al. 2020 , Wheeler 2012).
Behavioral Observations
Swifts are known to forage at heights of 20-150 m above buildings and trees (Steeves et al. 2014), but our observers recorded swifts foraging at higher approximate altitudes of greater than 250 m along one of our study routes (WPM personal observation, near a roost chimney). We lack finer resolution in our height class data because our highest class was “250 m and above” to avoid having to make our citizen scientist observers try to make more difficult assessments of greater heights. Smaller insects that swifts utilize as prey can be moved to these heights and/or concentrated by air movement and currents resulting from topographic surfaces and updrafts (Drake and Farrow 1989). Radio tracked Chimney Swifts in Guelph, Ontario foraged near industrial segments of the city that essentially create strong thermal currents (Wheeler 2012). Prey insects have been studied flying to heights > 900 meters above ground level in another study (Helms et al. 2016), and observations during the present study convince us that swifts may forage regularly at similarly great heights.
In terms of infrequently-described flight behavior, Chimney Swift utilize specific flock behaviors, especially during roost activity, like the three-dimensional trajectories in flocks of swifts noted by Evangelista et al. (2017).
Few autumn roost flocks were detected along our survey routes, no doubt because our observers found few large chimneys that would host fall roosts. In the exception located by the senior author, the roost flock built gradually in number of birds over the month of August, to a high of 122 birds on 16 August (WPM observations, this study).
Chimney Construction and Use by Swifts
Behavioral observations of Chimney Swift by colleagues in our region (but outside of our immediate study area) compelled these colleagues and ourselves to collect notes on behavior providing additional new or seldom-reported findings on aspects of chimney construction that may permit or enhance nesting by swifts. Previous studies show that swifts prefer chimneys of a size at least 25 cm in diameter (approximately 490 square cm), and other authors noted few chimneys smaller than this that were in use (Steeves et al. 2020). Many of the chimneys observed have ceramic flues at the uppermost end. In some case these flues extend only 0.5 meter into the overall length of the chimney, and not the entire length of the chimney. Swifts still utilize this chimney type - if the flue cross-section dimensions are large enough to permit entry and exit by swifts (see above). (Thomas Shultz, personal communication, 2018; WPM personal observation 2018). The senior author has observed Chimney Swifts entering and exiting ceramic flues that are approximately 30 centimeters by 30 centimeters in cross section and built into brick chimneys.
Acknowledgements
Thanks to the following cooperators who did monitoring routes on this project:
Jennifer Ambrose, Steve Baldwin, Carrie Becker,
Charlotte Catalano, Kathy Gallick, Erica Gerloski, Emilie Hunn, Karen Johnson, Beth
Kaplan, Rachelle Ketelhohn, Matt Mendenhall, William Mueller, Jean Prochnow,
Annika Roberts, Josie Roberts, Barbara Schwartz, Carl Schwartz, Robin Squier, Barbara
Todd, Carolyn Vargo, Tim Vargo, Vic Vargo, Anthony Watson, Jean Zachariasen,
and Norma Zehner.
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