Study of Cuckoos and Reed Warblers
Ian Wyllie
Plates 48-51
INTRODUCTION
In the spring of 1973 wildlife photographer Maurice Tibbies began
to make a film for television on the Cuckoo Cuculus canorus under a
two-year contract for the BBC and Time-Life Films. At a site close
to my home in Cambridgeshire a young Cuckoo, about two weeks
old, was found in the nest of a Reed Warbler Acrocephalus scirpaceus;
this was filmed by Tibbies and we subsequently found a further
twelve young of different ages and three Cuckoo eggs, all in Reed
Warbler nests. We had discovered a seemingly excellent site but it
was by then late June and the end of the Cuckoo's laying period.
After visiting various Cuckoo habitats all over Great Britain, Tibbies
decided that this site in Cambridgeshire offered the best opportunities for filming one of the most cryptic aspects of the Cuckoo's
behaviour—the egg-laying. This paper recounts some of the observations made on Cuckoos when Tibbies returned to the site in 1974
to complete the film.
Until 1922, when E. P. Chance published his book The Cuckoo's
Secret, there were several theories concerning the act of egg-laying.
The most generally accepted of these, based on sightings of Cuckoos
carrying eggs in their beaks, was that the egg was laid on the
ground, picked up in the beak and thus placed in the host species'
nest. Chance (1922, 1940) disproved this and showed that the
Cuckoo layed directly into the nest. As well as collecting scientific
data, which have since been analysed by Lack (1963) and Seel
(I973)> Chance made numerous assertions that we considered
sufficiently reliable to help us in our attempts to film the egg-laying.
In essence, these were as follows: only the male Cuckoo calls
'cuckoo' and only the female 'bubbles'; the female maintains a
territory and deters other females wishing to use that area; each
female parasitises one main host species; she locates the nests and
lays her eggs unaided by the male; she may visit the nest to determine its precise location prior to egg-laying; when a nest is selected
for egg-laying the female becomes determined to use that nest; if
a selected nest is destroyed or a suitable nest cannot be found she
may lay almost anywhere, even on the ground or in the nest of an
unsuitable host species; the egg can be retained in the oviduct for
several hours (but not indefinitely) until conditions for laying are
suitable; each female lays eggs which differ in size, colour and
markings from those of other Cuckoos, i.e. each female can be
identified by her eggs; the eggs are laid at intervals of not less than
IBril, Birds,6&: 369-378, September 1975]
3%
37°
Cuckoos and Reed Warblers
two days, i.e. eggs are not laid on consecutive days; eggs tend to be
laid in 'clutches' of about five, with a longer interval (than two days)
between each clutch; the egg is usually laid after midday following
a varying length of time spent by the female watching the host's nest
from a suitable vantage point; each egg is usually laid when the
host species is forming a clutch, but preferably when the host has
two eggs; the female 'glides' to the nest in hawk-like fashion when
about to lay; she lays directly into the host species' nest, usually
removing one egg; the time taken to lay an egg is extremely brief—
about nine seconds; cuckoos take no further interest in the host's
nest after egg-laying; where host species' nests are plentiful a
female may lay up to 25 eggs in one season.
Further investigations are required to verify many of these claims
but, for the purpose of our studies, they were accepted as true. It
should be noted that Chance's data were collected on Cuckoos
parasitising Meadow Pipits Anthuspratensis and that we were attempting to apply them to Reed Warbler-Cuckoos.
To film a Cuckoo laying an egg it was necessary to predict where
and when a particular female was going to lay. We knew from our
experience in 1973 that Reed Warblers were the main host species,
so a constant watch had to be kept on all Reed Warbler nests that
could be found in the area. When the first Cuckoo egg appeared
we could, according to Chance, expect that subsequent eggs would
be laid in the afternoons on alternate days in nests with one, two or
three eggs. Also, we could expect to see the bird watching or visiting
a chosen nest before an egg was actually laid.
The study site in Cambridgeshire consists of a number of small
Phragmites reed-beds, some of which are surrounded by bushes and
mature trees. A plan of the site (fig. 1) shows the area used by
Cuckoos with the positions and distances between the relevant
habitat features. The site was visited on 85 consecutive days from
28th April until 21st July, during which over 300 hours' field-work
were logged. Visits were usually made in the morning, in the evening
and all day at week-ends. Notes were kept on the Cuckoos' whereabouts and activities, on the contents and stages of development of
Reed Warbler nests, and on the eggs and young of Cuckoos.
FIELD OBSERVATIONS
A minimum of eight Cuckoos were in the area at some time during
the 1974 season. At least two were males which were frequently
heard calling against one another. Evidence for six females was
established from the variation in egg-colour by which we also
labelled them. These included Cuckoo P laying pink eggs similar to
those Of a Robin Erithacus rubecula; Cuckoo B laying brown eggs like
those of a Skylark Alauda arvensis; Cuckoo s laying a spotted egg like
Cuckoos and Reed Warblers
37i
Fig. 1. Sketch plan of Cambridgeshire site in 1974 showing distribution and order
of laying of eggs from six Cuckoos Cuculus canorus
that of a House Sparrow Passer domesticus; Cuckoo BL laying eggs with
a bluish tinge; Cuckoo G laying grey eggs similar to those of a Pied
Wagtail Motacilla alba; and Cuckoo L laying a very large grey egg.
Arrival
The site was first visited on 28th April when a single male Cuckoo
(identified by call) was in evidence, together with at least one
singing male Reed Warbler. The first female (identified by the
bubbling call) arrived one week later and was heard on 4th May
at 07.00 hours (the times given throughout are BST). The second
male arrived on 6th May and immediately began to sing against the
first, which resulted in much aggression and displaying between the
two. On 8th May both males were seen displaying to a female and
thereafter Cuckoos were frequendy seen in twos and threes and, on
one occasion, four were seen together. A calendar of Cuckoo
activities in 1974 is shown in fig. 2.
The first pair of Reed Warblers was seen on 12th May and the
first signs of nest-building were noticed on 18th May. The two male
Cuckoos were heard and seen daily until and including 23rd May,
but there was no further sign of an obvious female until 26th May,
by which time two Cuckoo eggs had been laid (see table 1). No
Cuckoos were heard or seen on 24th and 25th May although
Cuckoo p had almost certainly laid her second egg of the season in
the afternoon of 24th May. All the Cuckoos, therefore, had become
very secretive at the beginning of the laying period.
Cuckoos and Reed Warblers
372
Egg-laying
A female Cuckoo 'bubbled' in the morning of 26th May, and at
13.15 hours she took up position in a hedge about 20 metres from
a Reed Warbler nest containing three eggs in Southtrack reed-bed
(see fig. 1). At 14.15 hours the Cuckoo dropped with a crash into
the dead reeds about two metres from the nest. She emerged a few
seconds later to return to her previous position and within five
minutes, accompanied by a male Cuckoo, flew to South reed-bed.
To our surprise Cuckoo P had not laid and there was no further
sign of Cuckoos that day. The following morning Cuckoo p's diird
egg was found in a nest with two of the host's eggs in South reed-bed.
The fourth and fifth eggs of this particular bird were laid in nests
containing no eggs and they, too, were not discovered until the
morning after they were laid. By the end of May, Cuckoo P had
succeeded in laying five eggs, probably in the afternoons on alternate
days, without our having witnessed one of the layings.
From our brief notes made in 1973 it was apparent that three of
these five eggs had been laid in nests located almost exactly where
young Cuckoos had been found the year before. We decided, therefore, to pay rather more attention to sites where Reed Warblers had
been cuckolded in 1973.
One such site (West reed-bed in fig. 1) consisted of a small patch
of reeds surrounded by bushes, where both male and female Cuckoos
were often seen. By 2nd June the only pair of Reed Warblers using
this site had completed their nest, so a completely camouflaged hide
was constructed under the bushes about two metres away. On 3rd
June the warbler laid her first egg and at 15.00 hours, having given
no warning, Cuckoo p scrambled through the reeds, removed the
Reed Warbler's single egg and laid her own direcdy into the host's
nest. The whole operation took only 11 seconds and can be seen in
the film The Private Life of the Cuckoo.
Five other Reed Warbler nests at a suitable stage for egg-laying
by Cuckoos were also available on 3rd June, including two more
At feast one male present
At least two males present
A t least one female present
Egg-laying by Cuckoo P
Egg-iaying by Cuckoo B
Egg-laying by Cuckoo BL
Egg-laying by Cuckoo S
Egg-laying by Cuckoo G
Cuckoo eggs in nests
Cuckoo young in nests
Fledged young Cuckoos present
■ i : . ^ ^
, ■-
"T"...:,n
| »j,4::;~t;
30^
Apr
,.«1|
1
ED
1 ..
"
•> 11
|.*'.i,S«8KK«l
a
1
. .1
1 -„ „
31
May
-1
30
June
31
July
31
Aug
Fig. 2. Calendar of Cuckoo Cuculus canorus activities from observations at Cambridgeshire site, 1974
Cuckoos and Reed Warblers
373
Table I. Dates -when eggs of Cuckoos Cuculus canorus were laid, Cambridgeshire, 1974
Asterisks refer to Reed Warbler Acrocephalus scirpaceus pairs which were cuckolded
twice. The data in parentheses are assumed or estimated
Date
(22-5)
24-5
26.5
28.5
30.5
(1.6)
3-6
3-6
(6.6)
7.6
7.6
(11.6)
12.6
16.6
17.6
17.6
19.6
23.6
(27.6)
29.6
i-7
(2-7)
(3-7)
(6.7)
(6.7)
11.7
Time (BST)
Cuckoo
—
P
P
P
P
P
B
P
B
BL
B
S
B
G
G
B
BL
B
B
B
B
B
BL
B
BL
L
B
after
after
11.30
14.20
—
—
—
15.00
after 12.45
before 19.00
after 14.30
I7-I5
—
before 19.00
11.30-20.30
19.20
before 20.00
after 18.00
16.15
—
after
18.15
18.00
—
before 14.20
—
—
before 20.40
Reed Warbler
pair number
No. host's
eggs
35
4
25
14
(2)
2
2
0
0
(2)
1
1
(2)
12*
Sedge Warbler
7*
36*
33
2
11
12*
42
H
5
24
10
53
36*
7*
8
38
9
45
54
6
3
4
(2)
2
3
2
1
2
2
—
2
1
—
(0
—
2
Site
East
Southtrack
South
Withy
Bridge
River
West
Central
Typha
Southtrack
Central
Bridge
Northeast
Withy
Southtrack
South
Central
North track
Central
West
Northtrack
North
Southtrack
North
River
Southtrack
nests containing single eggs. One of these was used by Cuckoo B
for her second egg of the season, her first having been laid in the
nest of a Sedge Warbler Acrocephalus schoenobaenus about 500 metres
from the main reed-beds around the track (see fig. 1). This was
the only Cuckoo egg found in anything but a Reed Warbler nest
and suggests that Cuckoo p, having established a laying pattern in
the main reed-beds, had prevented Cuckoo B from using 'her' nests.
Cuckoo B had moved into the main reed-beds at the same time that
Cuckoo P laid her last known egg. (On the morning of 26th May
three Cuckoos were sighted at Central reed-bed, two apparently
chasing the other to River reed-bed. Although only one bird
'cuckoo'd', it was thought that this had been Cuckoo P and her
mate chasing the other male known to be in the area. However, it
could just as easily have been Cuckoo p driving Cuckoo B from the
main reed-beds with a male Cuckoo attempting to court both
females.)
Cuckoos and Reed Warblers
374
A regular pattern of egg-layings on alternate days was not recorded
after Cuckoo p's fifth egg. Nevertheless, the known dates of subsequent layings tended to be at multiples of two-day intervals. For
example, Cuckoo B laid on 3rd, 7th, 17th, 19th, 23rd, and 29th
June, and 1st and n t h July. As with Chance's observations,
undetected eggs could have been laid. (At least one egg was unlocated, the fledged youngster being seen on 18th August.) Cuckoo
B was filmed in the act of egg-laying on 23rd and 29th June (see
plate 50b).
Nest-robbing
An alarming number of Reed Warbler nests were predated throughout the season from 28th May onwards. Although the nests were
easier to find than usual owing to the poor growth of reeds caused
by the lack of spring rain, the only natural predator thought to steal
eggs was a pair of Jays Garrulns glandarius. Human disturbance by
bird-watchers, ramblers, fishermen and even egg-collectors was
responsible for the loss of some nests and eggs.
Predation of Reed Warbler nests by Cuckoos (apart from warbler
eggs taken during egg-laying) was recorded on nine occasions and
circumstantial evidence suggested that Cuckoos frequendy took
and ate eggs from nests on other occasions (see table 2).
On 7th June, at 17.05 hours, a Cuckoo was photographed taking
the only egg from a Reed Warbler's nest in South reed-bed (plates
49, 50). Ten minutes later Cuckoo s laid in a nest with four eggs
that had been incubated for four days in Central reed-bed. Although
Cuckoo s was possibly a young, inexperienced bird, the observations
suggest that the dominant female Cuckoo (at that time Cuckoo B)
attempted to prevent her from laying by robbing the egg from the
only suitable nest in the territory. No further eggs of Cuckoo s
were found, so she may have been driven out of the area or forced
to use some other host species.
Table 2. Host-nest predation by Cuckoos Cuculus canons, Cambridgeshire,
1974
Contents of
Date
Time (BST)
Nest number
nest
Predatioj
7.6
9.6
22.6
22.6
17-05
12.30
07.00
11.30
29.6
29.6
30.6
5-7
8. 7
13.00
18.00
11.30
15-15
16.30
25a
47
80
32
9°
75
75
86
103
1 egg
2 eggs
3 eggs
1 egg (infertile)
2 young
2 eggs
4 eggs
2 eggs
4 eggs
3 eggs
"
2
3
2
egg
eggs
eggs
young
2
2
2
4
3
eggs
eggs
eggs
eggs
eggs
Cuckoos and Reed Warblers
375
At 12.30 hours on 9th June (an assumed laying-day) the contents
of a nest with two eggs in Bridge reed-bed were robbed, accompanied by a series of excited bubbling calls. Several observers were
standing only about 50 metres from the nest and a hide had been
placed about four metres away in anticipation of laying. The
repeat-nest of this pair of warblers was also robbed on 17th June
when three eggs were taken. Furthermore, a female Cuckoo was
witnessed robbing a two-egg nest at this spot at 17.16 hours on
27th June 1973. It seemed that a female Cuckoo was repeatedly
victimising the Reed Warblers using this particular site.
Another nest-robbing incident is of special note as it involved the
predation of nestlings, an activity which has not been fully recorded
previously (see Chance 1940, p 167). Cuckoo p's sixth egg, laid on
3rd June in West reed-bed, mysteriously disappeared by n t h June
(it was possibly taken by egg-collectors). Two of the remaining
three Reed Warbler eggs hatched on 18th June, the third being
infertile. At n . i o hours on 22nd June two Cuckoos, presumably
females, were heard bubbling excitedly from the vicinity of the nest.
At 11.40 hours, after eight bubbling calls, a Cuckoo was seen eating
a young Reed Warbler. The four-day old nestling was held by the
abdomen, shaken in the same manner as a caterpillar, and finally
swallowed whole. An inspection of the nest and its surroundings
revealed no sign of the other young warbler but the infertile egg
was still present and intact. Cuckoo B was filmed laying in the
repeat-nest of this pair of warblers one week later.
Summary counts
By the end of the 1974 season, 26 eggs from six female Cuckoos had
been found and 27 were known to have been laid. One bird had
laid a minimum of 12 eggs, another six, a third four, and three
others one or two eggs each (see table 1). The laying period extended from 22nd May until the uncommonly late date of n t h
July. Male Cuckoos were heard from 28th April until 10th July
and females from 4th May to 14th July. (In 1973 the last female
was seen on 8th July.) Only two young fledged, the last being still
dependent upon the host species on 18th August.
There were 54 pairs of Reed Warblers and 136 of their nests were
found. Of these, 28 were deserted; 25 were cuckolded; 80 were
robbed; n produced fledged young (one, a Cuckoo); and 19 were
not followed up. Five Sedge Warbler nests were also found, one of
which was cuckolded, the others producing fledged young. An
unlocated Reed Warbler nest produced a fledged young Cuckoo.
DISCUSSION
Most of Chance's assertions on egg-laying by Cuckoos (pages 369-370)
376
Cuckoos and Reed Warblers
were supported by our 1974 observations. In some instances, however, our studies produced no evidence to support or refute his
claims. For example, the maximum number of eggs recorded from
a single Cuckoo in our study area in 1974 was twelve, whereas
Chance recorded up to 25 eggs laid by one bird (though under
experimental conditions). Also, while we cannot disprove Chance's
conclusion that only the male calls 'cuckoo', we believe that the
female may also do so on occasions. In the case of four of his points,
our evidence pointed to contrary conclusions, all of which, however,
may be related to the different host species (and very different
habitats) which were utilised in the different study areas. In particular, in our study area the Cuckoos did not have to cross large
open areas and may have had less difficulty in locating the nests of
their hosts. Unlike Chance's findings, mostly on Meadow PipitCuckoos, our observations of Reed Warbler-Cuckoos were that
(i) the male sometimes accompanied the female when egg-laying;
(ii) the female did not 'glide' to the nest but usually approached
through the reeds; (iii) the female did not visit the nest immediately
before laying to determine its precise location; (iv) cuckolded nests
were sometimes revisited and repeat-nests of cuckolded hosts were
occasionally cuckolded again.
If a successful breeding season at the site in 1973 was reflected
over the country as a whole, it may not be surprising that there were
as many as six female and at least two male Cuckoos in an area of
about 150 hectares in 1974. A full description of the eggs found in
1973 was not recorded but it was thought that they most resembled
the 1974 eggs of Cuckoo p. Since there was a tendency for her 1974
eggs to be laid in almost identical sites to those in 1973, this bird
may have been using the territory in both years. Alternatively, but
less likely, if the eggs of the female offspring resemble those of the
parent, Cuckoo P may have been a one-year old offspring of the
1973 female. A male Cuckoo in 1973 °f ten called from exposed
telegraph poles, but this behaviour was not noted in 1974 and it
seemed reasonable to believe that the dominant male of the previous
year had not returned.
Egg-laying by Cuckoos began on virtually the earliest possible
date (22nd May) for Reed Warbler nests and continued as late as
n t h July, after the males had apparently left the territory. Lack
(1963), using data recorded by egg-collectors G. Charteris and
A. E. Lees, indicated that Reed Warbler-Cuckoos did not apparently
lay after 8th July, despite the availability of nests. He suggested
that cessation of laying may be primarily determined by the
available food supply, although more research was necessary for
this hypothesis to be established. At the Cambridgeshire site in
1974 there was an abundant supply of Peacock Nymphalis io butter-
Cuckoos and Reed Warblers
377
fly larvae until the end of July, and the Cuckoos were seen feeding
on these from mid-June to 14th July. From records kept at Monks
Wood Nature Reserve, the peak numbers of adult Peacocks occurred
about one week later in 1974 compared with 1973. The Cuckoos'
departure from the site was also about one week later in 1974
and the two facts may be correlated.
In 1973 independent young Cuckoos were seen at the site from the
end of June onwards. In 1974, after much predation, the first
independent juvenile was not seen until 12th July, one day after the
laying of the last egg. Clearly, more research is required, but it
seems possible that the cessation of laying may be linked with the
appearance of fledged young Cuckoos within the breeding territory.
Cuckoos were known to rob nests on nine occasions extending
from 7th June until 8th July. Of the eight nests involved, 15 fresh
eggs, four deserted fresh eggs, and two four-day old nestlings were
taken. Circumstantial evidence, such as bubbling calls heard or
birds seen near the nests, suggested that 15% of the remaining 72
robbed nests were predated by Cuckoos.
There are three main reasons why adult Cuckoos may take eggs
and young from the nests of their hosts: ii) for additional food;
(ii) to force the hosts to re-nest, thus bringing their breeding cycle
in line with that of the Cuckoo; (iii) to prevent other female Cuckoos
laying eggs in suitable nests within the same territory. The predation
of nesdings supports the second of these reasons. Cuckoo B probably
took the young so that the warblers would build a repeat-nest
suitable for her own egg which was laid on 29th June. The fact
that two female Cuckoos were heard at the nest-site suggests that
both had designs on this particular pair of Reed Warblers, especially
as we know that a young Cuckoo was reared here in 1973. Again,
further research is needed to elucidate the reasons for nest-predation
by Cuckoos.
Future research
One of the greatest disadvantages in the field study of Cuckoos is
that the observer cannot positively distinguish between one bird
and another. Their secretive habits enable the birds to evade
observation for long periods and the observer can rarely be certain
of the sex of a particular individual. Although there is much evidence
to suggest that only the male calls 'cuckoo' and that only the female
'bubbles', it is by no means conclusive.
By marking Cuckoos from our breeding site over a period of years
and in such a way that an individual could be recognised in the
field, it is hoped to investigate the following: (i) the calls used by the
sexes; (ii) the incidence of polygamy, polyandry or promiscuity;
(iii) the size and maintenance of territories for each sex; (iv) egg-
378
Cuckoos and Reed Warblers
laying by the female unaccompanied by the male, at similar sites
each season; (v) the sex involved in egg-robbing and the reasons
behind such behaviour; (vi) adult Cuckoos' interest in their own
offspring; (vii) possible return by first-year birds to breed where
they were born and to parasitise the host species by which they were
reared; (viii) the longevity and breeding capacity of individual
Cuckoos; (ix) the duration of time spent at the breeding site,
especially in relation to food supply and egg-laying; (x) inheritance
of egg-colour; (xi) survival rates of young Cuckoos; (xii) the length
of time that the young remain dependent upon the host species and
the food taken by the young after reaching independence.
ACKNOWLEDGEMENTS
The work would almost certainly not have been undertaken but for the enthusiasm
of M, R. Tibbies and I am grateful to him for keeping notes on his observations
at crucial times when I was unable to visit the site. I am also indebted to J . L. F.
Parslow for his constructive criticism of an earlier draft of this paper.
SUMMARY
The paper reports observations on breeding Cuckoos Cuculus canorus in 1974 during
the making of a wildlife film on the species. The study site of about 150 hectares
in Cambridgeshire consists of a number of small reed-beds where 22 of 54 Reed
Warbler Acrocephalus scirpaceus pairs proved to be the main hosts. At least two male
and six female Cuckoos were found in the area between 28th April and 14th July,
and 27 eggs were laid between 22nd May and n t h July. The maximum number
of eggs laid by one female was twelve, the distance between the sites of laying of
first and last eggs being a little under 1 km. The eggs tended to be laid at two-day
intervals in the afternoons in nests with one, two or three eggs but occasionally in
nests with no eggs and once in a nest containing four incubated eggs. Only one
Cuckoo egg was laid in the nest of a different host species—a Sedge Warbler A.
schoenobaenus; this was possibly a forced departure from the preferred host, resulting
from territorial interference by another female Cuckoo.
Of 136 Reed Warbler nests found, 25 were cuckolded and at least 25% were
predated by Cuckoos, including one nest containing young Reed Warblers. It
was thought that the main reason for host-nest predation was to provide a continuity of suitable nests for the Cuckoos to use, although repeat-nests of warblers
known to have been previously predated by Cuckoos were not always cuckolded.
Cuckoos were filmed at the nest on three occasions, all of which can be seen in
the film The Private Life of the Cuckoo. Several still photographs were also obtained
of Cuckoos at Reed Warbler nests. The making of the film has stimulated further
research at the site and it is hoped that more information can be obtained by
studying experimentally marked Cuckoos over a number of years.
REFERENCES
CHANCE, E. P. 1922. The Cuckoo's Secret. London.
1940. The Truth about the Cuckoo. London.
LACK, D. 1963. 'Cuckoo hosts in England'. Bird Study, 10: 185-202.
SEEL, D. C. 1973. 'Egg-laying by the Cuckoo'. Brit. Birds, 66: 528-535.
Ian Wyllie, The Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS
P L A T E 48. Above, completed Reed Warbler Acrocephalus scirpaceus clutch containing in foreground slightly larger but similarly marked egg of Cuckoo
Cuculas canorus; below, using powerful leg muscles and hollow back, the young
Cuckoo ejects hosts' eggs eight to 36 hours after hatching (photos: Ian Wyllie)
P L A T E 49. Above, adult Cuckoo Cuculus canorus approaching nest of Reed Warbler
Acrocephalus scirpaceus through reeds; below, Cuckoo causes no d a m a g e to hosts'
nest as she clings to side in order to pick out an egg (photos: Jan Wyllie)
P L A T E 50. Above, a Cuckoo Cuculus canorus occasionally destroys the nest of
a Reed Warbler Acrocephalus scirpaceus by devouring the contents, causing hosts
to rebuild; below, when egg laying, a female Cuckoo holds egg of host in her
bill before quickly moving across open nest to drop own egg {photos: Ian Wyllie)
P L A T E 51- At 16 days old a young Cuckoo Cuculus canorus in nest of Reed Warbler
Acrocephalus scirpaceus already dwarfs its foster parent {photo: Ian Wyllie)