QuUodrcm et al • NESTING OF THE THORN-TAILED RAYADITO 
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central rectrices with a millimeter ruler. We 
recorded mass, tarsus, head-bill, and wing length 
for the young, using the same technique as for 
adults. We took measurements of all eggs, 
nestlings, and adults. Averages were calculated 
for each nest. 
The time required for nest construction was the 
period between the first record of nest material 
inside the nest box and the laying dale of the first 
egg. The incubation period was assumed to be the 
time between the last egg laid and hatching. 
Fifteen nests were removed, after the young had 
fledged, for a composition analysis. 
Statistical Analysis .—Data are expressed as 
mean ± standard deviation tSO). All nests are 
considered statistically independent because no 
two simultaneously active nests were in the same 
plot. We used Pearson correlation analysis for 
comparisons among nests, The criterion for 
statistical significance was v < 0.03. We used 
generalized linear models (GLM) with binomial 
response variables (occurrence of nest building, 
lav ing, and the presence of at least one successful 
fledgling) to evaluate the effect of distance to 
native forest on nest-site selection. Morphological 
measurements between adult and juveniles were 
compared using a paired /-test. The main results 
of our study and the previously published data for 
Chiloe Island were compared using a two-sample 
Welch's /-test. All statistical analyses were 
performed with R (R Development Core Team 
2008). 
RESULTS 
Thirty-four breeding pairs built nests in the 
boxes, and 23 (74%) began laying eggs. We 
estimated a nest construction time of 12.8 ± 
4.9 days (n = 23). Some pairs continued carrying 
building material to the nest during egg laying and 
■be beginning of incubation. Nest composition 
"as mainly pine needles and small twigs of trees 
and shrubs. The center of the nest contained 
leathers, mosses, epiphytes, herbs, and animal 
hair. Dry nest weight (41.6 « 1 4 g; range - 12.2- 
b9.8 g. n = 13) was not related to laying date (/• = 
~ 0.1, P = 0.58. n 15) or clutch si/e (r = 0.27. 
P = 0.33. n = 15). 
Nest construction began between 3 September 
and 23 November (median 4 Oct, n = 28). 
Laying occurred between 17 September and 18 
November with a median laying date of 15 
October (// = 24). Hatching of the first clutch 
occurred between 12 October and 8 December 
(median = 4 Nov, n = 22 ). Clutch size ranged 
from two to four eggs (3.3 ± 0.7, n = 22 ) with 
modal size of three eggs. Brood size at hatching 
ranged from one to four nestlings (2.9 ± 0.8, n = 
19) with a mode of three nestlings per nest. 
Laying occurred on alternate days with eggs 
measuring 18.6 ± 0.53 mm in length and 14.5 ± 
0.2 mm in width (n = 16). Mean egg volume was 
2.010 ± 92 mm' (» = 16) and mean mass was 
2.02 ± 0.14 g [n - 18). There was no association 
between mean egg volume and clutch size (r = 
-0.2. P — 0.44. n ~ 16) or laying date (/- = 0.06. 
P = 0.8. n = 16). Incubation lasted between 14 
and 18 days (15.8 ± 1.2, it = 20) with a modal 
time of 16 days. Fledging occurred at 21 days. 
There was no significant association between 
laying date and incubation period (/- = 0.01, P = 
0.96. n = 20). 
The lengths of 13-day old nestlings’ tarsus, 
wing, central rcctrix, and total body were 6%, 
48%, 75%, and 45% smaller than adults, 
respectively. However, adult body mass was 
I 1% lower than of nestlings at day 13 (Table 1). 
Brood size negatively affected the mass of chicks 
(/• = -0.74, P = 0.03, // = 8). and there was a 
positive correlation between adult mass and mean 
mass of nestlings (r - 0.52, P — 0.03. // - 16). 
There was no association between laying date and 
adult mass (/• = —0.03, P = 0.9, n = 18) or 
between laying date and nestling mass (/- = 0.3, P 
= 0.2, n - 18). 
Eleven of 25 active nests (44% ) were depre¬ 
dated before eggs hatched, and live (36%, 5/14) 
were depredated during the nestling stage. The 
total reproductive failure rate was 64%. Six pairs 
with depredated nests laid a second clutch; three 
were depredated during the egg stage and two 
during the nestling stage. Three successful pairs 
renested but these nests were not monitored. 
Considering all nests that hatched young [n = 19), 
89 ± 16% eggs hatched and 72 ± 44% chicks left 
the nest. 
We compared the results of our study with 
those obtained by Moreno et al. (2005) for a 
population of Thom-tailed Rayaditos breeding in 
a native forest at Chiloe Island. Chile (Table 2). 
Egg morphology and breeding success did not 
differ between populations. However, adults were 
heavier and took longer to build a heavier nest in 
the pine plantation near Constitucion. They also 
laid fewer eggs that were incubated over a longer 
period of time, and had fewer chicks per nest than 
documented for the Chiloe population. 
