reported an incidence of 50 percent. Our data exclude 
nests that had phoebe eggs perforated (probably by 
cowbirds) but in which no cowbird eggs were laid. Our 
nest records showed a higher rate of parasitism in central 
Illinois (29 percent) than our nests in the north, but the 
highest rate (33 percent) occurred in the northern 
counties of the southern region (north of Franklin 
County). On the other hand, in extreme southern Illinois 
south of Saline County where we studied 73 nests we did 
not find one parasitized nest. 
Of the 10 parasitized nests for which we had complete 
data, only 1 produced a phoebe (one phoebe plus one 
cowbird), 3 produced only cowbirds, and 6 failed 
completely, though not necessarily due to the parasitism. 
Incubation periods that we determined at several 
nests in different parts of the state were 15-17 days, 
usually 16, and nestling life was also 15-17 days. Nestling 
phoebes tend to remain in the nest as long as possible and 
often cling to the edge of the nest until their combined 
growth appears to exceed the capacity of the nest to hold 
them. In a nest with only two young the nestling stage was 
prolonged to at least 20 days. Upon fledging, phoebes 
usually fly well, but not with the competence of fledging 
swallows. A good first flight for the phoebe is probably 
important, because many phoebe nests are over water. At 
one nest we observed that at least some of the fledglings 
returned to the nest the first night. 
The nesting cycle for a first nesting, from onset of 
nest-building to fledging of young for a nest with five 
eggs, would be about 45 days: 9 days for nest building 
including the delay before the first egg is laid, 4 days for 
egg laying, 16 days for incubation beginning with the 
fourth egg, and 16 days of nestling life. Nest building can 
be at least as short as 7 days, reducing the cycle to 43 
days. In renesting the time could be reduced by 5 days, as 
refurbishing a nest can be done in 4 days or less (Fig. 22). 
Reducing the clutch size also reduces the cycle time by 1 
day per egg. Thus, the cycle could be only 38 days for a 
nest with 3 eggs. 
Even with such a long nesting cycle the possibility 
exists that a female could produce three broods in a year, 
as the egg-laying season is also long (Fig. 18). We suspect, 
however, that three broods would be exceptional. 
Estimating from the early laying dates (Fig. 18), the first 
fledglings could be expected about May 1 in southern 
Illinois, where we have seen grown young independent of 
their parents as early as May 26. At the other extreme, in 
central Illinois where the last eggs were laid July 2, nests 
with young could theoretically still be found as late as the 
first week in August. We have never found active nests 
later than July 18. We rarely observed postfledged young. 
Nesting success of phoebes was lower than we had 
expected in view of their protected nest sites (Table 4). In 
the northern and central regions, and in the northern 
counties of the south, success (30-43 percent of eggs 
fledged) in our samples of nests and eggs tended toward 
inverse correlation (r = — .763) with cowbird parasitism. 
Significantly, most of these nests failed during the 
26 
egg-laying or early incubation phases of the nesting cyc 
which is also indicative of cowbird interference. Becai 
none of the nests in extreme southern Illinois we 
parasitized by cowbirds, we expected higher success 
these nests, but they were even less successful than nests 
more northern populations (Table 4). The unsuccess 
nests in extreme southern Illinois failed about equa 
during the incubation stage and the nestling stage. T 
least successful nests and eggs (17 percent of eggs fledgs 
were the second nestings in extreme southern Illinois. 
We have too little data to quantify the causes of n 
failures. The cowbird was probably a significant factor 
all but the extreme southern counties (see aboy 
Hatchability of eggs in unmolested, carefully checl 
nests appeared to decline with latitude. In the nor 
hatchability was an amazing 97 percent, but in | 
central region it was only 83 percent. Eggs in early ne 
in extreme southern IIlinois showed a hatchability of 
percent, but only 71 percent of the eggs in second m 
hatched. Flooding of nests because of rising streams wa 
serious problem, especially in the south. Falling m 
were also probably a significant mortality factor, | 
such incidences cannot definitely be differentiated fr 
nest dislodgement by predators, human vandals, 
livestock. The effect is the same in any case. 
Nests in extreme southern Illinois appear to be mi 
more subject to predation than more northern nests. 
the south we saw evidence of nest predation from birds 
both eggs and young, and in at least one case 
predator was probably a blue jay. In central Ilir 
Musselman (1933) found a house wren (Troglod 
aedon) removing 4-day-old phoebes from their nest, 
in the north we saw one phoebe nest taken over by ho 
wrens, but do not know whether there was real predat 
in this case. Snakes — in one case a black rat sn 
(Elaphe obsoleta) (Ridgway 1914) — have also bi 
recorded as predators on phoebe nests (Bowie Hann 
personal communication). Death of nestlings from he 
infestations of lice or mites occurred in at least two n 
in the south (see also Hunter 1935), and the death of 
brood of phoebes appeared to coincide with road 
spraying of herbicides near the nest. 
Fall Migration 
As do many other species, phoebes bec 
particularly inconspicuous in late summer. One nes! 
population of phoebes that we studied at a rock bluff 
in Pope County disappeared entirely by July 8, tho: 
the last young had fledged only 9 days earlier. ; 
population had surely not migrated, but had prob: 
either dispersed or moved its center of activity. 7 
movement may be fairly typical, as most phoebes apf 
to quit their nesting areas in July. | 
We have seen independent young phoebes as earl 
May 26 and newly fledged young on July 18 in south 
Illinois, but there are no data to show when any of t] 
young birds or their parents begin fall migration, OF ¥ 
