Mass-dependent take-off ability in wintering great tits (Parus major): Comparison of top-ranked adult males and subordinate juvenile females

Abstract

When birds are attacked by predators, initial take-off is crucial for survival. Theoretical studies have predicted that predation
risk in terms of impaired flight ability increases with body weight. However, studies in which attacks were simulated, and
within-individual daily changes in body weight were used to test mass-dependent take-off outside migration period, have so
far failed to show an effect of mass on velocity. In this field study I compared the mass/velocity relationships of alarmed
adult male and juvenile female great tits, Parus major. Fattening strategies differ among members of the dominance-structured basic flocks of wintering great tits, and dominant
individuals often carry significantly less amount of fat reserves than subordinates. Since the range of body weight gain/loss
is the least among dominant males, it was expected that impaired flight ability is more likely in lower-ranked female great
tits. The results show that the birds differed significantly in their daily increase of relative body weight. Average daily
weight increase of adult males was 6.2%, while it was 12.2% in juvenile females. Males were faster than females at take-off
both at dawn and at dusk. Flight velocity of males did not differ significantly between dawn and dusk, whereas females took
off at a significantly lower speed at dusk than at dawn. The results suggest that the larger fat reserves of subordinate females
needed to increase their chances of overwinter survival probably place them at increased risk of predation.

Full text

Abstract When birds are attacked by predators, initial
take-off is crucial for survival. Theoretical studies have
predicted that predation risk in terms of impaired flight
ability increases with body weight. However, studies in
which attacks were simulated, and within-individual dai-
ly changes in body weight were used to test mass-depen-
dent take-off outside migration period, have so far failed
to show an effect of mass on velocity. In this field study
I compared the mass/velocity relationships of alarmed
adult male and juvenile female great tits, Parus major.
Fattening strategies differ among members of the domi-
nance-structured basic flocks of wintering great tits, and
dominant individuals often carry significantly less
amount of fat reserves than subordinates. Since the range
of body weight gain/loss is the least among dominant
males, it was expected that impaired flight ability is
more likely in lower-ranked female great tits. The results
show that the birds differed significantly in their daily
increase of relative body weight. Average daily weight
increase of adult males was 6.2%, while it was 12.2% in
juvenile females. Males were faster than females at take-
off both at dawn and at dusk. Flight velocity of males did
not differ significantly between dawn and dusk, whereas
females took off at a significantly lower speed at dusk
than at dawn. The results suggest that the larger fat re-
serves of subordinate females needed to increase their
chances of overwinter survival probably place them at
increased risk of predation.
Keywords Birds · Fat reserves · Flight ability · Predator
evasion
Introduction
Fat reserves in winter resident bird species are adaptively
adjusted to increased requirements through winter fatten-
ing (Helms 1968; King 1972; Lehikoinen 1987; Haftorn
1989). Maximizing fat levels in winter might be the opti-
mal strategy when survival depends on fasting capacity
which increases with fat reserves (Evans 1969; King
1972; Blem 1990). Many species of small birds, however,
maintain fat depots below their physiological capacity in
winter, which suggests a cost involved with excessive
levels of reserves (King 1972; Blem 1990). The main cost
is suggested to be predation risk (Suhonen 1993; Cuthill
and Houston 1997). Several theoretical studies predict
that predation risk increases with larger fat reserves and
that there is a trade-off between predation and starvation
risk (Lima 1986; McNamara and Houston 1987, 1990;
Hendenström 1992; Houston et al. 1993; Rogers and
Smith 1993; Bednekoff and Houston 1994a, b; McNamara
et al. 1994; Bednekoff 1996).
The risk of predation is expected to be mass-depen-
dent and to increase with fat reserves. There is indirect
empirical evidence of mass having an effect upon avian
predation risk, such as a reduction in mass or fat storage
in relation to an increase in perceived predation risk
(Gosler et al. 1995; Lilliendahl 1997). This maintenance
cost of body reserves (Clark and Ekman 1995; Houston
et al. 1997) suggests impaired flying ability, that is,
mass-dependent escape ability. Direct studies on the
effects of mass upon flight performance have, however,
been somewhat less consistent in their findings. Several
recent studies have focused on the maintenance costs,
and mass-dependent escape ability has been demonstrat-
ed (Marden 1987; Hendenström 1992; Witter and Cuthill
1993; Witter et al. 1994; Metcalfe and Ure 1995;
Kullberg et al. 1996, 1998, 2000; Lee et al. 1996; Lind
et al. 1999). However, studies in which attacks were
simulated, and within-individual daily changes in body
weight were used to test mass-dependent take-off outside
the migration period, have so far failed to show an effect
of mass on escape ability (Kullberg 1998; Kullberg et al.
Communicated by C. Brown
I. Krams (✉)
Department of Sciences, Daugavpils University, Vienibas iela 13,
5400 Daugavpils, Latvia
e-mail: krams@apollo.lv
Fax: +371-54-22890
Behav Ecol Sociobiol (2002) 51:345–349
DOI 10.1007/s00265-002-0452-8
ORIGINAL ARTICLE
Indrikis Krams
Mass-dependent take-off ability in wintering great tits (
Parus major
):
comparison of top-ranked adult males and subordinate juvenile females
Received: 21 March 2001 / Revised: 4 December 2001 / Accepted: 3 January 2002 / Published online: 8 February 2002
© Springer-Verlag 2002

1998; Veasey et al. 1998; Chai and Dudley 1999; see
also Brodin 2000). These studies have two common fea-
tures. First, the studies were done under laboratory con-
ditions where the captive birds usually decrease their
weight. Second, the birds used in the experiments were
usually males which possess high social status in the
wild.
Fattening strategies may differ significantly among
members of the dominance-structured social groups and
dominants often carry significantly less amount of re-
serves than subordinates (Ekman and Lilliendahl 1993;
Gosler 1996; Krams 1998a, b, 2000). Since the range of
body weight gain/loss is least among dominants, it may
be suggested that impaired flight ability is less likely in
higher-ranked male individuals.
To establish the true effect of mass upon escape ve-
locity, it is necessary to study the flights of birds belong-
ing to different social classes. In this field study, there-
fore, I compared the mass/velocity relationships of
alarmed adult males and juvenile females, which repres-
ent the top individuals and the bottom-ranked subordi-
nate members of the great tit, Parus major, basic flocks,
respectively.
Methods
This study was conducted near the town of Kraslava (55°53′N
27°11′E) in southeastern Latvia between the beginning of Novem-
ber and the middle of December in winters 1999 and 2000. All
field work was carried out in an area of summer houses enclosed
by rich mature forest of common spruce Picea abies and Scots
pine Pinus sylvestris with an understorey of rowan Sorbus aucup-
aria, common juniper Juniperus communis and common buck-
thorn Rhamnus catharica. During both study periods the average
day and night air temperature was similar and varied from –6
to +2C°. Snow was present on the ground and trees. Wind speed
varied from 0 to 1.0 m/s during experiments.
In winter, great tits are organized in a looser social system than
the majority of temperate parids. They belong to “basic flocks”
(Saitou 1978) composed of either several birds or just a pair. The
area used by a basic flock appears more like a home range than an
exclusive territory. Basic flocks readily intermingle when they
meet and utilize a communal area (Saitou 1979). The study area
was inhabited by several basic flocks of individually colour-ringed
great tits of known age and sex. Wing (maximum chord) and
tarsus were measured to 0.5 mm to give a measure of body size
(Garnett 1976). Dominance order was measured within a basic
flock using pairwise interactions between birds at feeders when
members of a local flock were not accompanied by neighbouring
conspecifics. To determine individual rank within each basic flock
I followed the procedures of Koivula and Orell (1988). The results
confirm previous reports that in the great tit males dominate
females and, within sex, adults generally dominate juveniles (e.g.
Saitou 1979; Krams 1998c).
To examine whether body mass impairs escape speed of indi-
viduals I recorded the behaviour of local adult males and juvenile
females, which are the top-ranked individuals and the bottom-
ranked birds, respectively. I used five adult males and six juvenile
females during the first winter and six adult males and five juve-
nile females during the second winter. Birds were caught individu-
ally by baited traps when they first arrived at dawn. Each individ-
ual was weighed by Pesola spring scales and then placed in a
holding box (10×10×10 cm) and alarmed upon release. After
3–4 days I repeated the procedure to obtain the evening weight of
each individual. This was done during the last hour of their daily
activity. I usually used mist nets to catch the birds in the evening.
Only one record was made per individual per each morning/eve-
ning. As soon as each bird was weighed I filmed its escape flights
by opening the holding box and releasing the bird. The data set in-
cludes on average 9.0±0.30 (mean±SE) flight trials recorded per
alarmed adult male at dawn, 10.0±0.30 flights trials per adult male
at dusk, 9.5±0.39 flight trials per juvenile female at dawn, and
8.6±0.43 flight trials per juvenile female at dusk. The flights were
filmed using a camcorder (Sony DCR-TR8000). The film was
played back on the VCR using a frame by frame facility to mea-
sure the time taken for each bird to fly horizontally up to a line
marked 40 cm from the head of the bird at the moment of escape.
Time taken to complete this distance was then converted into ve-
locity. Although the place of release differed from the place of
capture, the birds were familiar with the site of release since that
platform was often used as a feeding site. This familiarization was
required to ensure the birds were aware of the presence of thick
bushes of redcurrants Ribes spicatum at a distance of 3 m. The
feeding platform was placed 1 m above the ground and the birds
usually handled sunflower seeds in the bushes roughly at the same
height. To ensure sufficient replication I used medians of each in-
dividual bird (n=22) as the independent observation for statistical
testing. The birds were trained to come to the permanent feeders
in the territories when hearing a sound signal. Food at feeders was
supplied only during observational hours.
The positive relationship between size and social rank has been
reported in the great tits (e.g. Garnett 1976; Krams 2000). Consis-
tently, adult males were significantly larger than juvenile females
among the birds weighed when wing length was used as a measure
of size (mean and SE for males: 78.89±0.21 mm, mean and SE
for females: 75.21±0.30 mm, two-tailed t-test: t=10.23, df=20,
P<0.0001). Therefore, I controlled for size by calculating a body
weight index (BWI) for which the body weight was divided by the
third power of the wing-length (Ekman and Lilliendahl 1993;
Krams 1998a, b, 2000). Since extra fat may affect escaping behav-
iour (Witter et al. 1994; Kullberg et al. 1996; Lind et al. 1999),
wing-length should be of biological significance for fat reserves
(McNamara and Houston 1990). To find a relationship between
changing body weight and speed at take-off I calculated the rela-
tive increase of weight and the relative change of speed for each
individual.
Results
Condition of birds
At dawn adult males were significantly heavier than juve-
nile females (two-tailed Mann-Whitney U-test, U=48,
n1=n2=11, P<0.0001, Table 1). At dusk adult males were
also significantly heavier than juvenile females (two-
tailed Mann-Whitney U-test, U=45, n1=n2=11, P<0.0001,
Table 1). The males gained on average 2.29 g during their
activities while juvenile females gained 1.25 g (Table 1).
The birds differed in their daily increase of relative
weight: average daily weight increase of males was
6.2±0.53% (mean±SE) whereas the weight increase in fe-
males was 12.2±0.69%. Using the BWI, adult males and
juvenile females were found to be carrying roughly equal
reserves at the beginning of daily activities (two-tailed
Mann-Whitney U-test, U=5, n1=n2=11, P=0.74, Table 2).
At dusk the rates of BWI indicated significantly larger re-
serves of juvenile females than those of adult males (two-
tailed Mann-Whitney U-test, U=45, n1=n2=11, P=0.0014,
Table 2).
346

Mass dependence
Males were faster than females at take off both at dawn
(two-tailed Mann-Whitney U-test, U=19, n1=n2=11,
P<0.03, Table 1) and at dusk (two-tailed Mann-Whitney
U-test, U=32, n1=n2=11, P<0.0001, Table 1). If escape
performance is mass dependent, speed at take-off at dusk
would be expected to be lower than at dawn. Speed of
males at take-off did not differ significantly between
dawn and dusk (Table 1). In contrast, females took off
at a significantly lower speed at dusk than at dawn
(Table 1). The relative changes of body weight did not
affect the changes of speed in males while a negative re-
lationship between daily changes of body weight and
speed at take-off was revealed in juvenile females
(Fig. 1). So mass dependency on speed was found only
in females (Fig. 1).
Discussion
Some recent experimental studies have shown that daily
changes in body weight do not affect speed and angle of
take-off of attacked birds (Kullberg 1998; Kullberg et al.
1998; Veasey et al. 1998). In this study great tits per-
formed horizontal escape flights to the nearest cover. It
is known that members of dominance-structured flocks
may differ in their fattening strategies (Ekman and
Lilliendahl 1993; Gosler 1996; Krams 1998a, b, 2000).
Indeed, the birds differ in their body size and weight,
female great tits being significantly smaller and lighter
than males (Garnett 1976; Krams 2000). My experiments
revealed mass dependency on speed at take-off. How-
ever, it was found that natural body weight changes
affected subordinate females and dominant males in
different ways. While females were slower at dusk than
at dawn, the escape speed of males remained the same in
the morning as in the evening.
Weight changes in male great tits were of the same
magnitude as that of those birds used in earlier experi-
ments in which mass dependence on speed at take-off
was not revealed (Kullberg et al. 1998; Veasey et al.
1998). My results support the idea that mass increase up
to 10% may not influence the bird’s escape speed. In
contrast subordinate juvenile females gained significant-
ly larger body reserves than adult males during the day.
In winter air temperatures affect birds directly, by in-
creasing energy demand, and small birds have relatively
high energy requirements (Calder and King 1974). The
short period of daylight restricts available foraging time
and necessitates a long nocturnal fasting period. Under
conditions when food resources are limiting (Jansson
et al. 1981), subordinate individuals may avoid the risk
of starvation by carrying larger fat reserves as a buffer
against periods of high energy demand or food shortage
(Ekman and Lilliendahl 1993; Krams 1998a, b). How-
ever, some studies have shown that female great tits may
347
Table 1 Body weight and speed (means of medians for individual birds) at take-off of alarmed top-ranked adult males and subordinate
juvenile females at dawn and dusk
Dominants (adult males) Subordinates (juvenile females)
Dawn Dusk TnP
aDawn Dusk TnP
a
Mean SE Mean SE Mean SE Mean SE
Mass (g) 18.89 0.13 20.16 0.15 2 11 <0.01 16.45 0.22 18.74 0.18 0 11 <0.01
Speed (m/s) 3.84 0.05 3.77 0.06 12 11 >0.05 3.72 0.02 3.46 0.06 1 11 <0.01
aTwo-tailed Wilcoxon’s matched-pairs signed-ranks test
Table 2 Body weight index
(kg/m–3) of top-ranked adult
males and bottom-ranked juve-
nile females at dawn and dusk
(means of medians for individ-
ual birds±SE)
Dawn Dusk TnP
a
Mean SE Mean SE
Dominants (adult males) 38.47 0.29 41.14 0.52 0 11 <0.01
Subordinates (juvenile females) 38.70 0.59 44.09 0.60 0 11 <0.01
aTwo-tailed Wilcoxon’s matched-pairs signed-ranks test
Fig. 1 The effect of daily changes in relative body weight on
changes of relative speed of great tits at take-off. The relative in-
crease in body weight between dawn and dusk significantly affects
speed of escaping juvenile females (filled circles) (Pearson
r=–0.75, P=0.0078) while adult males (open circles) are not
affected (r=–0.53, P=0.092)

often be close to the upper level of their ability to with-
stand the harsh climate conditions occurring during cold
spells despite the increased energy intake (Hilden 1977;
I. Krams, unpublished data). This may be due to the thin-
ner plumage of juvenile females. As a rule male great tits
moult completely before the beginning of winter where-
as juvenile females may postpone moult. The contour
feathers provide virtually all of the insulation against
loss of heat (Brooks 1968; Dawson et al. 1983). There-
fore, the incomplete moult of many young females may
have made the insulation of their feather cover less ef-
fective (Middleton 1986) and larger fat reserves are
needed to compensate their higher overnight expendi-
tures.
In the morning both males and females carried similar
body reserves; however, males escaped faster than fe-
males. This supposedly was due to larger muscle mass of
males. At the end of the day males took off at the same
speed. However, juvenile females were significantly
slower at dusk than at dawn, which may make them an
easier target for predators. Since females took off more
slowly at dusk, foraging may be most risky towards the
end of activity day (Lima 1988). There is some evidence
that forest passerines experience a high relative preda-
tion risk especially during twilight hours when both noc-
turnal and diurnal predators are abroad simultaneously
(Mikkola 1983; I. Krams, personal observations). My re-
sults agree with theoretical ideas about impaired escape
performance when natural changes in body weight are
considered. However, the results presented show the im-
portance of dominance relationships and sex- or age-spe-
cific traits of social birds which cause considerable dif-
ferences in their fattening strategies. Since escaping
from a predator is a matter of life and death, leaner and
quicker birds should be at lower risk. However leaner
subordinate individuals may have difficulties acquiring
fat reserves before roosting while dominants can avoid
overnight starvation by displacing subordinates from
food sources according to the definition of social domi-
nance (Wilson 1975). Nocturnal fasting is definitely
more predictable than attacks of predators; therefore
subordinate juvenile females choose to avoid starvation
risk which is the proximate risk while top-ranked adult
males can afford to avoid both possible risks. However,
survival in tits is usually rank-dependent and dominants
survive better than subdominant individuals (Ekman et al
1981; Koivula and Orell 1988). Mass-dependent take-off
and an increase in exposure time when acquiring fat re-
serves (Krause and Godin 1996) may also be important
in explaining decreased winter survival in subordinate
tits.
Acknowledgements I thank Peter A. Bednekoff, Jukka Suhonen,
Raivo Mänd, Ilva Everte, Christiaan Both and an anonymous ref-
eree for valuable comments which greatly improved the manu-
script. Financial support was received from the Science Council of
Latvia. All animal manipulations reported were carried out in ac-
cordance with the legal and ethical standards of the Republic of
Latvia.
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