Daytime and nighttime activity at a breeding colony of Great Blue Herons in a nontidal environment

Abstract

It is generally admitted that in coastal areas, herons of the genus Ardea adjust their foraging time according to the tidal cycle. However, to what extent do tides control the herons' daily rhythm of activity? To answer this question, we present the day and night activity patterns of Great Blue Herons (Ardea herodias) arriving to feed their young at a heronry located in a nontidal environment in southern Quebec. Herons were about half as active at night as during the day. Therefore, although significantly less than diurnal activity, nocturnal activity was not negligible, and consequently the tide cycle is not the only factor controlling the daily rhythm of the herons' activity. Those breeding pairs that were most active during the day were no more or less active at night. Diurnal activity was more closely correlated with the number of young that fledged than was nocturnal activity. Thus, night activity was not necessarily important for the survival of young herons, but it could be explained by other factors such as the greater availability of certain prey at night.

Full text

Daytime and nighttime activity at a breeding colony of Great Blue Herons
in a nontidal environment
RAYMOND MCNEIL
AND
REJEAN BENO~T
DPpartement de sciences biologiques, UniversitP de MontrPal, C.P.
6128,
succursale
A,
MontrPal, (QuPbec),
Canadu H3C 3J7
AND
JEAN-LUC DESGRANGES
Environnement Canada, Service canadien de la Faune, C.P.
10100,
Ste-Foy, (QuPbec), Cunada
GIV
4H.5
Received April 15, 1992
Accepted November 30, 1992
MCNEIL, R., BENOPT, R., and DESGRANGES, J.-L. 1993. Daytime and nighttime activity at a breeding colony of Great Blue
Herons in a nontidal environment. Can. J. Zool. 71: 1075
-
1078.
It is generally admitted that in coastal areas, herons of the genus Ardea adjust their foraging time according to the tidal
cycle. However, to what extent do tides control the herons' daily rhythm of activity? To answer this question, we present
the day and night activity patterns of Great Blue Herons (Ardea herodias) arriving to feed their young at a heronry located
in a nontidal environment in southern Quebec. Herons were about half as active at night as during the day. Therefore,
although significantly less than diurnal activity, nocturnal activity was not negligible, and consequently the tide cycle is not
the only factor controlling the daily rhythm of the herons' activity. Those breeding pairs that were most active during the
day were no more or less active at night. Diurnal activity was more closely correlated with the number of young that fledged
than was nocturnal activity. Thus, night activity was not necessarily important for the survival of young herons, but it could
be explained by other factors such as the greater availability of certain prey at night.
MCNEIL, R., BENO~T, R., et DESGRANGES, J.-L. 1993. Daytime and nighttime activity at a breeding colony of Great Blue
Herons in a nontidal environment. Can. J. Zool. 71
:
1075
-
1078.
I1 est generalement admis que les herons du genre Ardea ajustent leurs periodes d'alimentation selon le cycle des marees
dans les regions catikres. Cependant, jusqu'a quel point les marees contralent-elles le cycle d'activite des herons? Pour
repondre a cette question, nous presentons les patrons d'activite diurne et nocturne de Grands Herons (Ardea Iterodias) revenant
a leur nid pour nourrir leurs jeunes dans une colonie du sud du Quebec, situee dans un milieu non soumis a l'action des
marees. Les herons ont CtC moitik aussi actifs la nuit que le jour. Quoique significativement inferieure a l'activiti diurne,
I'activite nocturne n'a donc pas kt6 negligeable et, en consequence, le cycle des marees n'est pas le seul facteur a contraler
le cycle journalier d'activiti des herons. Les couples les plus actifs de jour n'ont kt6 ni plus ni moins actifs de nuit. L'activite
diurne a semble avoir une influence plus grande que I'activite nocturne sur le nombre de jeunes a I'envol. L'activite diurne
n'a donc pas CtC particulikrement importante pour la survie des jeunes, mais elle peut s'expliquer par d'autres facteurs, tels
la plus grande disponibiliti de certaines proies la nuit.
Introduction
A
number of species of Ciconiiformes, in particular the
ardeids, are known to feed almost as commonly at night as
during the day (Bent 1963; Hancock and Kushlan 1984; McNeil
et al. 1993). In coastal habitats, it is generally recognized that
herons of the genus Ardea, like other ardeids in intertidal
environments, adjust their foraging activities according to food
availability and predictability influenced by the tidal cycle, so
as to be in their feeding area at times when fishing or preying
is most likely to be successful (Hancock and Kushlan 1984).
However, to what extent do tides control the daily rhythm
of herons' activity? The daily feeding cycle of the Great Blue
Heron (Ardea herodias) appears to be influenced by the tidal
cycle in coastal areas (Dennis 1971; Krebs 1974; Brandman
1976; DesGranges 1981; Drapeau 1982; Black and Collopy
1982). However, in freshwater areas far inland in Oregon, the
Great Blue Heron seems to display a strictly diurnal pattern in
feeding its young, typical of many avian species, with peak
periods of feeding in the mornings and evenings (Horvath and
Moholt 1986). The study by Horvath and Moholt (1986) sug-
gests that Great Blue Herons adjust their feeding cycle depend-
ing on whether or not they are in a tide-controlled environment,
but unfortunately,
it
did not include a representative sample of
24-h observations
(04:OO
to 22:OO PST).
Nevertheless, Horvath and Moholt (1986) appear to be the
only researchers to have dealt with the daily feeding cycle of
Great Blue Herons in nontidal areas. The main purpose of this
article, with samples distributed over 24-h observation sessions,
is to compare the day and night timing of arrivals (hereafter
called activity) of Great Blue Herons at the nest site in a river-
ine lake near MontrCal, Quebec, to see if they feed their young
nocturnally in a nontidal environment as do Grey Herons (Ardea
cinerea) (Marion 1984; van Vessem and Draulans 1986, 1987).
This study also assesses the relative importance of night- and
day-time feedings to young herons aged 4 weeks or older.
Study area and methods
This study was carried out in the Montreal area, at the Great Blue
Heron colony on ile (island) Saint-Bernard on the southern edge of
lac Saint-Louis (45
"23'40"N, 73 "45'30"W) in Quebec. Lac Saint-
Louis is a riverine lake, formed by a natural widening of the St.
Lawrence River. This heronry contained 169 occupied nests (i.e.,
with either eggs or chicks) and the area observed corresponded to the
northern tip of the colony. Nests sampled were about 8 -25 m above
the ground.
Data were collected during observation sessions about 4 days apart
between 4 June and 3 July, 1987, from a 7-m tower located about
100 m from the closest nests. The eight observation sessions each
lasted 24 consecutive h, from 05:00 to 05:00
(EDT)
the following
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CAN.
J.
ZOOL.
VOL.
71,
1993
=
Night
04/06 10/06 12/06 14/06 18/06 22/06 24/06 28/06
Day
I
month
Time (EDT)
FIG. 2. Average number (+_SD) of day and night arrivalslh at the
28 nests observed during 24-h periods.
FIG. 1. Variation in the average number of arrivalsl(2 h) at the
28 nests observed during 24-h periods.
TABLE 1. Suearman's rank correlations
(P
5
0.05) between mean
day. Two observers were always present in the morning (05:00 to daily, diurnal, and nocturnal activity per 2-h period and fledging suc-
09:00), in the evening (17:00 to 21:00), and at night from 21:00 to cess at the 28 nests observed during 24-h periods at ile Saint-Bernard
05:00. On several occasions, only one observer was present during Activity
the remaining periods (09:00 to 1790). Nighttime observations were Fledging
made using a MK303A
(X
60
000)
light intensifier (Star-tron Tech- Daily Diurnal Nocturnal success
nology Corporation, Pittsburgh, Pa.). During the night, one of the
observers constantly scanned the nest area by means of the intensifier.
Both at night and during the day, the approach call of adults normally
signalled the arrival of an adult at its nest. There is no doubt that most
nocturnal arrivals at the nest were to feed the young. In many cases,
the actual prey given to the young was not seen at night, but the
characteristic movements of the heron's neck as the heron regurgi-
tated food to the young were detectable.
The present data are incidental to a study of the patterns of arrival
and departure of Great Blue Herons nesting at ile Saint-Bernard
(Benoit 1991). For that reason, observations on the feeding grounds
were not feasible. Data collection at the colony began when the young
herons were 3-4 weeks old. Observations ceased when the young
fledged. We recorded arrival and departure times to the nearest minute
and the identity of the nests involved. However, we did not use depar-
ture data from these nests in the present study, since these movements
were not always detectable at night. The main flight directions of
herons at the colony were already known, so the observation tower
was located to minimize its impact on the movements of herons
around the colony. The adults did not appear to be disturbed by the
presence of observers during observation sessions.
In analyzing nocturnal and diurnal activity of the colony as a whole
and of selected breeding pairs, we confined ourselves to the number
of arrivals per 2-h period (12 periods for each 24-h observation session)
at a group of 28 nests that we could observe both by night and by day.
The diurnal period of activity included the dusk and dawn hours.
Results
Some 1220 arrivals were recorded at the 28 nests during
the eight 24-h periods, 951 during the day and 269 at night.
This corresponds to an average of 14.9
+
6.6
(x
)
SD)
arrivalsl(2 h) and 8.4
(
_+
3.9) arrivalsl(2 h) during the day and
night, respectively, for all 28 nests (Fig. 1). The most intense
diurnal activity took place at dawn (23.4
f
8.1 /(2 h)) and
at dusk (16.4
f
6.6/(2 h)). The lowest activity was in mid-
morning and during the afternoon (12.6
+
4.2/(2 h)), and in
the middle of the night (8.4
f
3.9/(2 h)).
In general, the frequency of diurnal arrivals for the entire
sampling period was significantly greater than that of nocturnal
arrivals (Fig. 2; Student's paired t-test, t
=
7.712; df
=
27;
P
<
0.001). The number of diurnal arrivals (Mann-Whitney
U-test, U
=
0;
P
<
0.05), but not the relative magnitude of
Daily activity
1
.OO 0.80 0.55 0.3
1
Diurnal activity 1.00 ns 0.39
Nocturnal activity 1.00 ns
Fledging success
1
.OO
diurnal activity (U
=
3;
P
>
0.05), was greater after 14 June.
Nocturnal activity clearly took place throughout the entire
sampling period.
Spearman's rank correlation tests (P
5
0.05; Table 1) on
the frequency of arrivals per 2-h period for the whole sample
(n
=
28) showed that, of diurnal (r,
=
0.80) and nocturnal
activity (r,
=
0.55), it was diurnal activity that appeared to
form the greater component of the total day's activity (24 h).
The activity of a pair during the day showed no correlation
with the pair's activity at night (r, ns).
Diurnal activity was correlated with the fledging success
of pairs (r,
=
0.39; n
=
28), but there was no correlation
between nocturnal activity and fledging success (Table 1 and
Fig. 3). In addition, for pairs feeding the largest broods
(4-5 young), the frequency of diurnal activity (0.55
+
0.11)
was greater than that of nocturnal activity (0.29
+
0.12)
(Wilcoxon signed-ranks test; T
=
3; n
=
19;
P
<
0.05), but
no such difference was observed in pairs rearing smaller
broods (T
=
7; n
=
9;
P
>
0.05).
Discussion
Total diurnal and nocturnal arrivals were more numerous
at sunrise and sunset. This largely corresponds to what was
reported by Horvath and Moholt (1986) and van Vessem and
Draulans (1986, 1987). The frequency of nocturnal activity at
the colony of ile Saint-Bernard accounted for, on average, 63
%
of the activity observed during the preceding diurnal periods
for the eight sampling sessions. The intensity of nocturnal
activity on 1 day was even greater than that of diurnal activity.
Therefore, although significantly less than diurnal activity, noc-
turnal activity was not negligible. The activity pattern of the
Great Blue Heron was similar to that of the Grey Heron,
though Owen (1955) observed a colony of Grey Herons in
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MCNEIL ET AL.
1077
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0
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o
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a
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FIG. 3. Relationship between the number of fledged young and the
mean number of arrivalsl(2 h) during the day and night, respectively.
which activity was more nocturnal on certain days, while
van Vessem and Draulans (1986) observed that these Euro-
pean herons were sometimes just as active at night as during
the day.
The Great Blue Herons most active during the day were not
necessarily those that were least active at night and vice versa.
On the whole, nesters spent more time and effort feeding their
broods during the daytime period. Daytime activity was greater
after 14 June probably because young herons require more and
more food as they grow.
According to Hancock and Kushlan (1984), the nocturnal
activity of Great Blue Herons in the colony occurs more com-
monly in marine environments owing to the tide cycle, and
Brandman (1976) expected that nocturnal activity in the con-
tinental region would
be
half as intense (50%) as during daylight.
This is what we observed for the colony at ile Saint-Bernard.
Consequently, the tide cycle is not the only factor controlling
the herons' daily rhythm.
The number of daytime feeding arrivals was significantly
higher in nesting pairs with larger broods. These results con-
cur with those of van Vessem and Draulans (1986) and Sullivan
(1988). They also bear out the predictions of Lack (1968)
regarding the influence of food on fledging success. Brandman
(1976) observed that the number of feedings given to young
Great Blue Herons during the day and at night in coastal areas
varied with the size of the brood, but he did not assess the
importance of the contribution of night feeding to the fledging
success of herons. Our results also support the idea of Drent
and Daan (1980) and Collopy (1984) who explained differences
in fledging success between broods by variations in the parental
ability to feed the young. For the Great Blue Heron in particu-
lar, Sullivan (1988, p. 224) concluded that "broods fledging
4 or more nestlings were raised by adults which were able to
provide food more frequently than most pairs." The amount
of food brought to the nest during the very 1st week after
hatching must be essential and of great influence on the final
breeding success of herons. Unfortunately, we do not have
data for the period between hatching and the age of 3 -4 weeks,
and we have not measured the actual quantity of food brought
to the young. However, this ability of good providers is borne
out in our study by the number of arrivals on the nest to feed
young aged 4 weeks or older.
As mentioned above, no observation was made on the feed-
ing grounds. However, the decrease in activity observed at
night in this colony did not necessarily mean that adults were
less active on feeding sites at night. Indeed, Black and Collopy
(1982) observed greater use of feeding sites by Great Blue
Herons at night than during the day. Furthermore, Draulans
and van Vessem (1985) reported ,that Grey Herons are more
abundant on feeding sites at night, but they referred to the
winter period and not to the breeding season, and secondly
they dealt mainly with the situation on a fish farm. For Grey
Herons, the vertical migration of fish toward the surface dur-
ing the night would explain the nocturnal activity observed by
Draulans and van Vessem (1985) in artificial basins. These
authors suggested that this migration is also likely in natural
conditions and might influence use of feeding areas. In addi-
tion, diurnal horizontal feeding migrations are very clearly
marked in many freshwater fishes that live in midstream by
day and enter creeks at night where there is usually a rich
invertebrate fauna on which they feed (Nikolsky 1963, p. 254).
In lac Saint-Louis, fish are known to make a nocturnal horizontal
migration between the pelagic zone of the lake and the shore-
line marshes (Ferraris and Lessard 1988). Finally, it may also
be that herons forage at night on sites and on prey that are not
used during the day. The diet of Great Blue Herons, like that
of many other ardeids (Hancock and Kushlan 1984), includes
frogs, worms, and insects that could be more abundant or
more active, and thus more available, at dusk and during dark-
ness. In contrast to activity in coastal marine environments,
which is linked to the rhythm of tides, the nocturnal activity
of Great Blue Herons in a riverine nontidal lake environment
is possibly related to the activity of prey.
Acknowledgements
This study was financed partly by the Canadian Wildlife
Service, and partly by the Province of Quebec Society for the
Protection of Birds through an agreement with Employment and
Immigration Canada. We thank Pierre Drapeau and Alain Leduc
for help with statistical analysis, Diane Dauphin, Sharon David,
Linda Herwood, Stacy Hewitson, Colin McGowan, Louis
Panneton, and Jean Rodrigue for assisting in fieldwork, and
Janine van Vessem for valuable suggestions for improving the
manuscript.
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