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Reproductive Success and Habitat Selection in Black-
crowned Night-Herons (Nycticorax nycticorax) in a
City Park
Author(s): Victoria M. Hunt
Source: The American Midland Naturalist, 175(2):168-182.
Published By: University of Notre Dame
DOI: http://dx.doi.org/10.1674/0003-0031-175.2.168
URL: http://www.bioone.org/doi/full/10.1674/0003-0031-175.2.168
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RESEARCH-ARTICLE
Reproductive Success and Habitat Selection in Black-crowned
Night-Herons (Nycticorax nycticorax) in a City Park
VICTORIA M. HUNT
1
Department of Conservation and Science, Lincoln Park Zoo, Chicago, Illinois 60614
ABSTRACT.—Black-crowned Night-Herons (BCNH; Nycticorax nycticorax) increasingly colonize
urban areas, demonstrating they consider the value of such habitat to outweigh the risks.
However it is unclear if cities support reproductively successful populations of BCNH. To begin
to address this question, I evaluated if a park in Chicago, Illinois, provided suitable breeding
habitat or was an ecological trap for a colony of approximately 400 BCNH. Nest densities
were 217 nests/ha in 2010 and 315 nests/ha in 2011, which were higher than nest densities
observed in North American BCNH colonies in natural habitats. Ratios of young to active nests
were 1.22 in 2010 and 0.76 in 2011, similar to ratios observed in nearby BCNH colonies. Within
the park BCNH selected between two neighboring habitat patches. Logistic regression was
used to predict habitat patch selection as a function of colony size and year. In the model the
probability of selecting a larger, more exposed habitat patch versus a smaller, more secluded
habitat patch, increased with colony size. This trend in habitat patch selection demonstrated
behavioral flexibility which may have facilitated successful colonization of a human-modified
landscape. The findings support the conclusion that in 2010 and 2011, an urban park in
Chicago supported a locally endangered BCNH population and was not an ecological trap.
INTRODUCTION
As urbanization progresses it becomes increasingly important to understand the role
of human-modified landscapes as wildlife habitat (Dearborn and Kark, 2010). Some wildlife
populations thrive in urban areas, but for other populations urban areas are ecological
traps in which wildlife choose low-quality habitat over high-quality habitat based on faulty or
incomplete information (Battin, 2004). Black-crowned Night-Herons (BCNH; Nycticorax
nycticorax) increasingly occupy urban areas (e.g., Kelly et al., 2006) but it is unclear if such
areas provide suitable breeding habitats because anthropogenic disturbance potentially
disrupts BCNH breeding (Tremblay and Ellison, 1979; Parsons and Burger, 1982). Therefore
it is unclear whether BCNH colonies which select urban areas for breeding have found
refugia, or have unwittingly fallen into ecological traps.
BCNH have colonized human-modified landscapes including cities and suburbs for
decades (Hothem and Hatch, 2004; Kelly et al., 2006). For example a suburban colony in
Penngrove, California has been active since the 1930s (Kelly et al., 2006). Other suburban
colonies in California include West 9th St. of Santa Rosa and the Napa State Hospital campus
(Kelly et al., 2006). BCNH also colonized Alcatraz Island in California, a popular tourist
attraction with over a million visitors per year (Hothem and Hatch, 2004; Kelly et al., 2006).
Examples of urban colonies can be found on the East Coast of the United States as well;
BCNH colonized an urban estuary in New York Harbor (Craig et al., 2012) and forage on and
around Staten Island, New York (Bernick, 2004). Erwin et al. (1991) describe a colony of more
than 300 BCNH pairs in Baltimore Harbor, Maryland and posit the colony benefits from
proximity to urban lights which attract fish. Some colonies of egrets and herons (family
1
Corresponding author current address: Department of Ecology and Evolutionary Biology, University
of Illinois at Chicago, Chicago, Illinois 60607; e-mail: vhunt5@uic.edu
Am. Midl. Nat. (2016) 175:168–182
168
Ardeidae), including BCNH, become so successful in urban areas that they are considered
nuisances because of their odors, guano, loud vocalizations, and the perception they pose
a health risk (Grant and Watson, 1995; Parkes et al., 2012).
BCNH choose to colonize urban areas in a variety of scenarios but it is unclear if such
areas are suitable for breeding. Evidence suggests urban areas become ecological traps for
some BCNH colonies when colony abandonments result from nest tree removal by private
residents in suburbs (Kelly et al., 2006), similar to an oft-cited ecological trap in which
grassland bird nests are mowed over (Best, 1986; Schlaepfer et al., 2002; Battin, 2004; Sih et
al., 2011). Even in the absence of direct management against breeding colonies, BCNH are
susceptible to human disturbance which can result in nest abandonment, nest failure,
behavioral changes in nestlings, young mortality, and inhibition of egg laying (Tremblay and
Ellison, 1979; Parsons and Burger, 1982; Kelly et al., 2007). Ferna´ndez-Juricic et al. (2007)
determined nestling BCNH increase vigilance and decrease maintenance behaviors such as
grooming in response to disturbance by pedestrians and boats. Consumption of environmental
contaminants is also of concern for BCNH foraging in industrialized areas (Newman et al., 2007;
Levengood and Schaeffer, 2010; Padula et al., 2010). In Illinois where this study occurred, the
selection of urban areas by BCNH may in part reflect their limited alternatives. BCNH have been
listed as endangered in Illinois since 1977 based on small population size, a history of decline,
and extensive wetland habitat loss (Illinois Natural History Survey and Illinois Department of
Natural Resources, 2011; Illinois Endangered Species Protection Board, 2011).
Differences in behavioral flexibility can provide insight into why some wildlife populations
thrive under human induced rapid environmental change (HIREC), including urbaniza-
tion, and others do not (Sih et al., 2011). Generally the more behavioral flexibility exhibited
by a wildlife population, the better their chance of initially surviving HIREC, and of sub-
sequently adapting to novel conditions (Sih et al., 2011). For example urban Northern
Goshawks (Accipiter gentilis), which have successfully colonized cities throughout Europe,
demonstrate specialized hunting techniques that use novel features in the urban environ-
ment, such as hunting from perches on television aerials (Rutz, 2006). Urban Northern
Goshawks also display flexible habitat use that includes both patches of green space and built-
up habitat, spending most of their time in green spaces (parks) where they nest and making
frequent hunting forays into surrounding built-up habitat (Rutz, 2006).
In this study my objective was to conduct a quantitative assessment of reproductive success
and habitat patch selection, and use this assessment to consider whether or not Lincoln
Park in Chicago, Illinois, was an ecological trap for a breeding colony of BCNH. To this end
the results of regular monitoring efforts were used to: (1) determine how reproduction in
the colony compared with colonies in natural habitats, and (2) assess behavioral flexibility in
terms of habitat patch selection in an urban park.
METHODS
STUDY AREA
The study took place in Lincoln Park, Chicago, Illinois (41u5591.9599N, 287u37956.1299W).
Lincoln Park contains 485 ha of landscaped public-use park habitat. The park is located
4 km north of the Chicago Loop, the downtown business district (Fig. 1). Human use of
the park is high throughout the year, particularly from April to August when thousands of
people visit and attend events including concerts, fairs, organized picnics, and carnivals. For
example multiple charity events for cancer awareness co- occurred with this study, each of
which brought more than a thousand participants to Lincoln Park (for example, Pancreatic
Cancer Action Network, 2010).
2016 HUNT:URBAN BLACK-CROWNED NIGHT-HERONS 169
The study area consisted of two distinct habitat patches in Lincoln Park: a small 0.18 ha
grove of several trees in a pond (hereafter the grove) and a 0.67 ha tree-lined avenue located
40 m south of the pond (hereafter the avenue). Figure 1 illustrates the spatial arrangement
of the two habitat patches within the park. The circumstances of the grove changed over the
FIG. 1.—Map of the study area showing two Black-crowned Night-Heron habitat patches (avenue and
grove) in Lincoln Park, Chicago, Illinois, in 2011. Base map data obtained from the Illinois Natural
Resources Geospatial Data Clearing House, University of Illinois at Urbana-Champaign
170 THE AMERICAN MIDLAND NATURALIST 175(2)
course of the study due to restoration efforts, which involved draining (November 2008)
and refilling the pond (summer 2011) and stocking the pond with fish (July 2011). The grove
and drained pond were fenced off from the public in 2009 and 2010 while the pond was
empty. Wildlife are increasingly challenged to adapt to rapid environmental changes caused
by humans (Sih et al., 2011) and the changes that occurred in Lincoln Park are prime
examples. Furthermore some restoration efforts unintentionally become ecological traps
(Robertson et al., 2013). Therefore restoration efforts in Lincoln Park created an opportunity
to observe BNCH responses to HIREC, as well as to anthropogenic disturbance in general.
The grove habitat patch contained linden (Tilia americana), ash (Fraxinus americana), and
white birch trees (Betula papyriferan). Although many small saplings less than 3 m tall grew in
the grove, it was estimated only seven trees were sturdy enough to be selected by the BCNH
to support nests. The grove’s overgrown understory included buttonbush (Cephalanthus
occidentalis), blackhaw (Viburnum prunifolium), red dogwood (Cornus sericea), and riverbank
grape (Vitis riparia). Understory characteristics of BCNH breeding areas are important
because undergrowth can harbor and conceal predators (Baker et al., 2015). BCNH in
Lincoln Park in 2007-2009 nested in the grove, therefore restoration of the pond did not
result in immediate relocation of the colony.
South of the grove, the avenue habitat patch was characterized by approximately 50
regularly spaced ash and linden trees. A popular public path bisected the avenue and
mowed turf grass grew beside the path. There was no understory in the avenue. BCNH were
never observed nesting in the avenue before 2010.
Motion-triggered wildlife cameras (Bushnell Trophy Cam HD Max Game Camera;
Bushnell, Overland Park, Kansas) were used to determine which species of mammalian
predators occurred in the study area. Cameras revealed raccoons (Procyon lotor), coyotes
(Canis latrans), and foxes (Vulpes vulpes and Urocyon cinereoargenteus). Raccoons and coyotes
occur in high densities in the Chicago region (Prange et al., 2003; Gehrt et al., 2010).
Additionally domestic dogs (Canis lupus familiaris) and cats (Felis catus) frequented the park,
often unattended by their owners. Aerial predators including Peregrine Falcons (Falco
peregrinus), Cooper’s Hawks (Accipiter cooperii), American Crows (Corvus brachyrhynchos), and
gulls (family Laridae) were observed during the BCNH breeding season. On one occasion
a Ring-billed Gull (Larus delawarensis) was observed eating a BCNH chick in the avenue in
2010; this was the only direct observation of predation in the study area.
CENSUSES
I conducted 192 censuses: 113 in 2010 and 79 in 2011. During each census the number of
BCNH in three age classes (adults, juveniles .1 y old, and young born in the concurrent
breeding season), locations of BCNH (grove, avenue, or other), number of active (occupied)
nests, and number of trees containing active nests were recorded.
With assistance from three trained observers, daily censuses began when adult BCNH
arrived at the park (2 April 2010 and 30 March 2011) and continued until all young had
dispersed (20 September 2010 and 12 August 2011). Censuses were taken from the ground
using binoculars, beginning 2–3 h after sunrise (Hoefler, 1980; Ralph et al., 1981). Censuses
were unconstrained by time, requiring up to 2 h when the population was at its peak. Where
birds occur at high density, such as in BCNH colonies, birds are not necessarily detected in the
earlier portions of such censuses and total time spent conducting censuses is important for
detecting all birds (Slater, 1994).
During each census the entire way around the pond, which contained the grove habitat
patch, and down and around the complete length of the avenue habitat patch were walked.
A boardwalk allowed me to census the grove from a distance of less than 100 m from three
2016 HUNT:URBAN BLACK-CROWNED NIGHT-HERONS 171
cardinal directions: north, east, and south (Fig. 1). In the avenue I was able to observe from
directly beneath nest trees and therefore viewed the nests from a distance determined by the
nest height, which I estimated to be approximately 10 m in most cases. Movement of BCNH
during censuses was minimal and consisted primarily of nest building and courtship. To
avoid double counting I did not count BCNH that startled and flew into parts of the colony
that had not yet been surveyed (Levengood et al., 2005).
ANALYSIS OF REPRODUCTIVE SUCCESS
Peak abundance of active nests can be reliably determined from a single count
conducted during peak nesting season because of the conspicuous nature of active BCNH
nests (Kelly et al., 2007). The nests are conspicuous for two main reasons. First, nests are
active for long durations because BCNH incubate eggs for approximately 24 d (Levengood
et al., 2005) and nestlings remain in the nest for several weeks after hatching. Second,
BCNH deposit copious quantities of guano and prey items under active nests (Bent, 1963;
Kelly et al., 2006). In Lincoln Park, BCNH also deposited egg shells, prey items, and sticks
beneath active nests. BCNH young are loud and parents are frequently observed actively
feeding hatchlings, making active nests all the more obvious (Kelly et al., 2006). I counted
active nests daily and was confident I determined the peak abundance because of the
conspicuous nature of the nests. Active nests included all nests occupied by adult or young
BCNH. Density of active nests was determined by dividing the peak abundance of active
nests by spatial area of the associated habitat patch (0.18 ha for the grove and 0.67 ha for
the avenue).
The ratio of young to active nests, or young to nesting pairs, is frequently used to
quantify reproductive success in colonial birds including BCNH (e.g., Tremblay and
Ellison, 1979; Hoefler, 1980; Crouch et al., 2002; Hothem and Hatch, 2004; Levengood et
al., 2005; Kelly et al., 2007). However there is no commonly accepted definition of young.
Young may include eggs, nestlings, and fledglings (Crouch et al., 2002), or it may only
include fledglings of a certain age, e.g., 7d(Kellyet al., 2007), 15 d (Levengood et al.,
2005), 28 d (Tremblay and Ellison, 1979), or 35 d (Hoefler, 1980). As soon as they were
visible from the ground young were counted, therefore my definition of young included
fledglings and nestlings. Nestlings fledge at until approximately 2 w of age and attain
flight at around 6 w of age (Levengood et al., 2005).
To estimate the number of young produced by the colony, I used the peak abundance of
young as determined by daily censusing. This peak count of young was actually the
minimum possible produced by the colony; the count excluded young that died before the
peak abundance census and young born after the peak abundance census. Young were
unlikely to have dispersed before the peak abundance censuses on 14 June 2010 and 8 June
2011 because BCNH disperse at approximately 58 d of age (Levengood et al., 2005) and
peak abundances of young occurred less than 58 d after the first young hatched.
Because of the endangered status of BCNH in Illinois and under advisement of the
Illinois Department of Natural Resources observation methods involving touching BCNH
(e.g., banding) were avoided (Maggie Cole, pers. comm.). I was unable to use the Mayfield
Method to determine nest success or similar methods involving following the fate of specific
nests (Mayfield, 1975) because eggs in the nest could not be observed. BCNH in Lincoln
Park nested approximately 10 m up and were only viewable from the ground. I did not have
equipment necessary to view or photograph nests from above; therefore, I could not record
the fate of specific eggs. Use of hydraulic lift vehicles (Crouch et al., 2002) should be
considered in future study efforts.
172 THE AMERICAN MIDLAND NATURALIST 175(2)
FINE-SCALE HABITAT PATCH SELECTION
The close proximity of two habitat patches (grove and avenue) and my ability to census
the entire BCNH colony daily presented an opportunity to investigate whether the colony
exhibited behavioral flexibility in terms of fine-scale habitat patch selection in response to
fluctuations in colony size. If BCNH were to exhibit behavioral flexibility in terms of habitat
patch selection, this was of interest because it may have aided the colony in adapting to
urban living (Sih et al., 2012).
To determine the effect of colony size on habitat patch selection, I used logistic regression
wherein the habitat patch selection of an individual BCNH was the binary dependent
variable of interest. Habitat patch selection was determined for the two mutually exclusive
habitat patches (avenue and grove), where habitat patch referred to the cluster of trees
occupied by an adult BCNH during a given census. Habitat patch selection was determined
for each censused BCNH in the subset of censuses collected prior to and including the peak
population censuses. Therefore census dates included were 2 April 2010 to 16 June 2010
and 30 March 2011 to 10 May 2011, which included 43 censuses in 2010 and 30 in 2011.
These censuses represented 5314 habitat patch selections by BCNH in 2010 and 3195 in
2011. I restricted censuses taken after the population peaks because at that time BCNH
migrated away from the study area and their habitat patch selections could no longer be
determined. A batch of censuses, taken in 2010 for the ,2 w period before I realized BCNH
were selecting the avenue habitat patch and began conducting censuses there, was also
restricted (Fig. 2).
Two predictor variables in the logistic regression were included. The first predictor was colony
size (n
total
), defined as the number of adult BCNH in the colony determined via census. The second
predictor was year (y), a categorical variable with two levels: 2010 and 2011. Procedure GLM in the R
statistical environment was used to perform the logistic regression, employing a binomial link
function (R Development Core Team, 2009). To determine which model was best supported by
census data AIC values were used (Akaike, 1974).
FIG. 2.—Stacked areas of counts of adult and young Black-Crowned Night-Herons at two habitat
patches (avenue and grove) in Lincoln Park, Chicago, Illinois. (A) Counts in 2010 from arrival on 2
April 2010 to departure on 20 September 2010. (B) Counts in 2011 from arrival on 30 March 2011 to
departure on 12 August 2011
*The first census in the avenue in 2010
2016 HUNT:URBAN BLACK-CROWNED NIGHT-HERONS 173
RESULTS
REPRODUCTIVE SUCCESS
BCNH nested in very high density in Lincoln Park, with 217 nests/ha in 2010 and 315
nests/ha in 2011. Some trees contained as many as 13 simultaneously active nests. The adult
BCNH colony size peaked at 251 in 2010 and at 397 in 2011. Despite 58%larger colony size,
the colony produced fewer young in 2011 compared to 2010. Peak abundance of young was
180 in 2010 and 160 in 2011 (Fig. 2). The ratios of young to active nests were 1.22 in 2010
and 0.76 in 2011.
In 2010 although many BCNH pairs initially began nesting on the grove, only one pair
remained there for the entire breeding season and they did not produce young. The other
146 active nests were in the avenue in 2010. In 2011 all 211 active nests were in the avenue.
After young BCNH were able to fly, they frequently left the avenue for the grove before
dispersing from the park, which is reflected in censuses of the two habitat patches (Fig. 2).
Young BCNH, less than 1 y old, were observed using the grove to practice flying and
foraging, in addition to roosting and congregating in small groups. Therefore in Figure 2,
which shows censuses of the grove and avenue habitat patches, the BCNH young observed in
the grove were not born there but were simply censused there while roosting, foraging, etc.
The wandering behavior of small pods of young BCNH at the end of the summer breeding
season is typical for this species (Semenchuk and Federation of Alberta Naturalists, 1992).
FINE-SCALE HABITAT PATCH SELECTION
Four models with all combinations of two predictors (colony size and year) and
interaction between predictors were considered (Table 1). The model with the most
support from the census data, as determined by AIC (Akaike, 1974), was given by
pavenue *ncolony|y
where p
avenue
was the probability of selecting the avenue habitat patch, n
colony
was number of
BCNH in the colony and ywas a categorical variable for year (2010 or 2011). Coefficients for
n
colony
,yand the interaction between n
colony
and ywere all significant (P ,0.001).
TABLE 1.—Comparison of candidate logistic regression models with binomial link function.
Probability of selecting the avenue habitat patch (p
avenue
) is a function of some combination of two
predictors and their interaction: colony size (n
colony
) and year (y). Sample size consists of 5314 habitat
patch selections in 2010 and 3195 in 2011. Census dates included were 2 April 2010 to 16 June 2010 and
30 March 2011 to 10 May 2011. AIC scores identified the model with the most support from census data
Model Description Equivalent degrees of freedom DAIC
p
avenue
,n
colony
3yProbability of selecting the
avenue is a function of colony size, year,
and the interaction between
colony size and year.
40
p
avenue
,n
colony
+yProbability of selecting the avenue
is a function of colony size and year.
3 133
p
avenue
,n
colony
Probability of selecting the avenue
is a function of colony size
2 248
p
avenue
,yProbability of selecting the avenue
is a function of year
2 4365
174 THE AMERICAN MIDLAND NATURALIST 175(2)
In both 2010 and 2011, it was more probable that a BCNH would not select the avenue
(and would therefore select the grove) at low colony sizes and would select the avenue at
high colony sizes (Fig. 3). For each increase in n
colony
of one BCNH, the odds of selecting the
avenue increased 1.05 times (determined from the antilog of the coefficient for n
colony
,
0.053). Interaction between year and colony size was such that the odds of selecting the
avenue were 59 times greater in 2011 than in 2010, controlling for n
colony
and interaction
between yand n
colony
(antilog of the coefficient for y, 4.08).
DISCUSSION
REPRODUCTIVE SUCCESS
BCNH in North America in natural habitats typically nest in high densities such as 62
nests/ha (Bent, 1963) and 88 nests/ha (Hoffman and Prince, 1975). Table 2 lists more
examples and associated details. Nevertheless densities observed in Lincoln Park, 217 nests/
ha and 315 nests/ha in 2010 and 2011 respectively, were markedly high, although for
example, in Europe and Asia higher nest densities have been reported (Kazantzidis et al.,
1997; Nam et al., 2007). Davis (1986) observed BCNH nested with only one pair per tree in
95%of cases on Clark’s Island, Plymouth, Massachusetts, and Bent (1963) also found that
the majority of nest trees had only one nest per tree in Cape Cod Bay, Massachusetts.
In California Crouch et al. (2002) observed some trees containing 15 BCNH nests, which was
similar to densities observed in Lincoln Park (13 active nests per tree). Bent (1963) reported
surveying 854 trees, only two of which had 13 and 14 nests respectively; the remaining
852 trees surveyed contained fewer nests.
Nesting in high density may be an adaptation to life in an urban environment as has been
found in Eurasian Tree Sparrows (Passer montanus) and House Sparrows (Passer domesticus)
(Møller et al., 2012). Supporting this hypothesis, Kelly et al. (2006) described a suburban
nest site at West 9th St. in Santa Rosa, California, in which BCNH nested in very high
density: approximately 200 active nests were located in two sub-colonies in the median
FIG. 3.—Logistic regression model of the probability of selecting the avenue habitat patch as a function of
colony size in 2010 (grey line) and 2011 (black line). Samplesizeconsistsof5314habitatpatchselectionsin
2010 and 3195 in 2011. Census dates included were 2 April 2010 to 16 June 2010 and 30 March 2011 to 10
May 2011. Dotted lines indicate 95%confidence intervals and symbol size scales with number of
observations. The logistic regression equation for 2010 is given by pavenue~1
1ze{{6:54964z0:05347|ncolony
ðÞ
. The
logistic regression equation for 2011 is given by pavenue ~1
1ze{{2:46818z0:02855|ncolony
ðÞ
2016 HUNT:URBAN BLACK-CROWNED NIGHT-HERONS 175
TABLE 2.—Black-crowned Night-Heron nest density (nests/ha) in Lincoln Park, Chicago, Illinois, compared with values from literature, sorted by year
of study
Area surveyed Nests/ha Year Location (Source) Habitat
16.2 ha 124 1910 Cape Code Bay Massachusetts (Bent, 1963)
1
Wooded area, sand hills.
1.4 ha 88 1971 Monroe County, Michigan (Hoffman and Prince, 1975) Wood lot.
185 m 3141 m 79 1996 Napa State Hospital, California (Kelly et al., 2006)
2
Hospital campus with lawns,
dorms and sports fields.
265 m 3215 m 12 2004 Penngrove, California Residential area.
280 m 360 m 149 1995 Brooks Island, California Natural island habitat, no bridge
(boat access only).
106 m 336 m and 22 m 311 m 478 2001 West 9th St., California Residential area, in the median of a 4 lane street.
0.67 ha 217, 315 2010, 2011 Chicago, Illinois (This study) Tree lined avenue in an urban
city park.
1
Colony size given as 0.125 mi by 0.5 mi, containing 2000 pairs, which I converted to 2000 nests
176 THE AMERICAN MIDLAND NATURALIST 175(2)
of a four lane street measuring 106 m by 36 m and 11 m by 22 m in 2001. Nesting in
high density is adaptive if fecundity increases in a density-dependent manner due to an
allee effect in which social stimulation is requisite for reproductive success (Davis, 1993).
Alternatively nesting in high density is maladaptive if there is an overabundance of urban
nest sites but inadequate food, as has been documented in urban European Kestrels (Falco
tinnunculus) (Sumasgutner et al., 2014). More nests does not necessarily equate increased
reproduction; Bennetts et al. (2000) determined reproductive parameters were negatively
associated with the number of Little Egret (Egretta garzetta) nests in a heronry, presumably
due to competition for suitable nest sites.
Ratios of young to active nests in the Lincoln Park colony, 1.22 in 2010 and 0.76 in 2011,
were similar to ratios observed in other BCNH colonies in North America (Table 3). The
colonies closest to the study area are in Lake Calumet, 30 km south of Lincoln Park. Ratios
of young to active nests in Indian Ridge Marsh, a hemi-marsh in Lake Calumet, were 1.74 in
2002 and 2.22 in 2003 (Levengood et al., 2005). Therefore I observed smaller ratios of young
to active nests in Lincoln Park than were observed at Indian Ridge Marsh. However a BCNH
colony nesting in cottonwoods (Populus deltoides) in the vicinity of an active steel mill near
Indian Ridge Marsh had ratios of young to active nests similar to those I observed: 0.52 in
2002 and 1.27 in 2003 (Levengood et al., 2005). Mean number of young produced per nest
attempt observed in California in Central San Francisco Bay, San Pablo Bay, Russian River
and Laguna de Santa Rosa (1.28, 0.79 and 0.78 respectively) were also similar to ratios I
observed (Kelly et al., 2007). Ratios observed in Lincoln Park were smaller than those
observed in Horicon and Mead Wildlife Areas in Wisconsin (1.98 fledged young per pair)
(Hoefler, 1980) and on multiple undisturbed islands in Que´bec (1.5, 1.3 and 2.1) (Tremblay
and Ellison, 1979). Although the values observed in Lincoln Park were similar to values
obtained in nearby natural habitats, they may signal reproduction that is lower than necessary
to sustain the population over the long term (Henny, 1972; Levengood et al., 2005).
The observed 58.2%increase in colony size between 2010 and 2011 likely resulted from
an influx of adult BCNH from another nesting site, not from intra-colony reproduction. The
increase in colony size in Lincoln Park came in the form of BCNH in adult plumage, which
supported the hypothesis that the influx resulted from immigration because acquisition
of adult plumage takes several years (see descriptions of immature and breeding plumages in
DeVore et al., 2004). Therefore the BCNH that constituted the influx were not young from
the previous year that had hatched in Lincoln Park. Large fluctuations in BCNH colony sizes
due to immigration, emigration, and colony abandonment are common (Hoefler, 1980;
Gross and Siefken, 2007; Kelly et al., 2006).
A possible source of the BCNH influx I observed was identified. Shortly before the Lincoln
Park colony experienced an influx, a BCNH colony was at least partially abandoned in Lake
Calumet (Maggie Cole, pers. comm.). The colony had nested at Lake Calumet with colony
sizes documented since 1984 (Levengood et al., 2005) and declined from 1600 adult BCNH
in 1992 to fewer than 600 adult BCNH in 2000 (Marcisz et al., 2005). Vetted census data
submitted to eBird, an online database hosted by Cornell Lab of Ornithology, supports the
hypothesis that BCNH left Lake Calumet and joined the Lincoln Park colony in 2011
(Cornell Lab of Ornithology, 2013). The peak colony size recorded in Lake Calumet in 2010
was 113 BCNH compared with only 24 BCNH in 2011 (Cornell Lab of Ornithology, 2013).
FINE-SCALE HABITAT PATCH SELECTION
I determined selection of habitat patches by BCNH depends on colony size and year,
therefore demonstrating behavioral flexibility. Behavioral flexibility has been implicated in
the success of wildlife under conditions of HIREC (Sih et al., 2012); therefore, it is hopeful
2016 HUNT:URBAN BLACK-CROWNED NIGHT-HERONS 177
TABLE 3.—Ratios of Black-crowned Night-Heron young to active nests in Lincoln Park, Chicago, Illinois, compared with values from literature, sorted by
publication year and year of study
n
nests
Young/Active nest Year Location (Source) Habitat
18 0 1975 Gros Pe` lerin, Que´bec (Tremblay and Ellison, 1979)
1
Uninhabited island. Colony was visited before egg laying began.
46, 64 1.5, 1.3 1975, 1976 Gros Pe`lerin, Que´ bec Uninhabited island.
43 2.1 1975 Ile Bruˆle´, Que´bec Uninhabited island.
66 0.5 1976 Ile Bruˆle´, Que´bec Uninhabited island. Colony was visited before egg laying began.
82 1.98 1978, 1979 Horicon and Mead Wildife
Areas, Wisconsin
(Hoefler, 1980)
2
Wildlife Areas associated with wetlands.
2187 0.74 1990-2002 Alcatraz Island, California (Hothem and Hatch, 2004)
3
Island 2 km north of San Francisco. Tourist attraction.
1285* 1.28 1993-2005 Central San Francisco Bay,
California
(Kelly et al., 2007, and habitat descriptions in Kelly et al., 2006)
4
Intensive urban development interspersed with wetlands, and several small islands,
national parks.
259* 0.79 1993-2005 San Pablo Bay, California A large wetland complex and restored pasture and salt evaporation ponds. Some
colonies nest in residential areas.
170* 0.78 1993-2005 Russian River and Laguna
de Santa Rosa, California
Cultivated bottomlands, forested hillsides along a
river and 8100 ha of wetlands, grasslands and woodlands.
48, 55 1.74, 2.22 2002, 2003 Indian Ridge Marsh,
Illinois
(Levengood et al., 2005)
5
Hemi-marsh in a polluted, former industrial area.
12,17 0.52, 1.27 2002, 2003 Inland Steel, Indiana Vicinity of an active steel mill, cottonwood tree grove.
147, 211 1.22, 0.76 2010, 2011 Chicago, Illinois (This study)
6
Urban park habitat with high human use.
1
Young defined as surviving to 28 d
2
Authors reported young per pair and I converted pairs to nests. Fledglings defined as surviving to 35 d or flying
3
Fledglings 15 d from first hatched egg in nest and sample size refers to monitored nests. Range of young/active nest ratios for different study years was
0.46 - 1.27
4
Young were at least 7 to 15 d old. *Sample sizes were reported for nest survivorship estimates. See Kelly et al. (2007) for the method they used to calculate
number of young produced per nest attempt
5
Young are defined as having survived to 15 d
6
Young are defined as fledglings and nestlings visible from the ground
178 THE AMERICAN MIDLAND NATURALIST 175(2)
that BCNH displayed flexibility in this study. BCNH typically display high nest site fidelity,
reusing nests in 86%of cases (Davis, 1986), making this demonstration of behavioral flexibility
all the more remarkable. The propensity to select the avenue habitat patch in 2011 may have
been influenced by the tendency to reuse nests (Davis, 1986) or learning, e.g., BCNH learned
the previous year that at high colony sizes the avenue provided more nesting sites.
The logistic regression model I developed supports the conclusion colony size affects
habitat patch selection, but it does not explain why. I hypothesize BCNH selected habitat
patches to strike the optimal balance between avoiding overcrowding and nesting in
adequate density for social stimulation for breeding (Davis, 1993). Before 2010 the BCNH
colony was smaller (approximately 30 adult BCNH in 2007) and nested exclusively on the
grove. The grove may have been easier to defend at small population sizes, but overcrowd-
ing made it unsuitable at large population sizes. Considering the grove contained only
approximately seven suitable nest trees and the trees were relatively small and short
compared to trees in the avenue, it seems likely the BCNH moved to the avenue when the
colony would have been too crowded in the grove (Burger and Gochfeld, 1993). The grove
and the avenue were close together and therefore it is likely the same predators accessed
both sites. However the grove may have been a more defensible nest site with better visibility,
making vigilance more effective. Alternatively, if larger colony size attracted more predators,
perhaps the understory of the grove concealed those predators (Baker et al., 2015) therefore
rendering the grove more dangerous at higher colony sizes.
That BCNH selected nest sites in a busy, urban area such as Lincoln Park demonstrated
the BCNH perceived the value of these nest sites to be greater than risks perceived from
anthropogenic disturbance. However the question remains: is the urban environment an
ecological trap for BCNH? One factor in whether a site is an ecological trap is if the
wildlife are able to read and respond to novel cues in their environment, and wildlife that
exhibit behavioral flexibility typically perform better in novel situations such as those
associated with urbanization (Sih et al., 2012). Results from the logistic regression model I
present suggest BCNH show behavioral flexibility in fine-scale habitat patch selection to
adjust for colony size fluctuations. This observation, coupled with the observation that
productivity in the colony was similar to that of colonies in nearby natural environments,
supports the conclusion that for this colony over the 2 y time period I investigated, a city
park served as a refuge and not as an ecological trap, and behavioral flexibility may have
played a role in the colony’s success. This approach consisting of assessment of
reproductive success and behavioral flexibility using fine-scale habitat selection could
contribute to evaluations of whether or not other urban bird colonies have fallen into
ecological traps.
Acknowledgments.—I thank two anonymous reviewers for detailed suggestions that improved this
manuscript. Dr. Joel Brown, Dr. Chris Whelan, and Dr. Steve Thompson provided helpful editorial
guidance. Carolyn Mazan, Danielle Brottman, and Cari Jones assisted with field work. T. Emiko
Condeso provided me with some of the data I used to compare nest density, from the atlas of Kelly et al.
(2006), which is cited in the text. Field work was funded by the Davee Foundation and Lincoln Park
Zoo. Data were used with permission from Lincoln Park Zoo.
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SUBMITTED 20 OCTOBER 2014 ACCESSED 6SEPTEMBER 2015
182 THE AMERICAN MIDLAND NATURALIST 175(2)
