Clutch size

The number of eggs laid varies between individuals within the species and with the time of year, with later broods generally containing fewer eggs:

Number of broods with 3

1971

1972 I

1973 3

1974

1975 I

1976

1977 1983

Percentage of total

The percentages of unhatched eggs and dead young in pied flycatcher nests during the 1970s were as follows:

N u m bur

r'.'r urihaiehud

dead

of broods

young

1970

33

8.4

7.4

1971

23

9.6

O.iS

1972

30

7.4

X.7

197.1

30

16.0

6 5

1974

23

19.6

5.«

1975

24

8.9

I7.,\

1976

24

10.2

5.S

1977

21

7.9

19 5

1983

30

7.9

1.2

Average

26

10.7

8.2

After a peak in 1973 74, the proportion of unhatched eggs fell significantly. This probably reflected the banning of chlorinated hydrocarbons in countries used by the pied flycatchers en route to their winter quarters, namely Portugal in 1972 and Spain in 1974. The high number of fatalities amongst young in 1975 and 1977 was due to the very low average temperatures (13- I7°C) during the period when the young birds were growing which meant a poor supply of insects. Parents were unable to find enough food for their young. By contrast, the average temperature in the same period in 1974 was I8-25°C.

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