body mass gain
Growth trajectories were significantly different between the two brood size groups (Table 1). As expected, nestlings in the enlarged broods grew slower during their first 20 days of age (Fig. 1). The pattern was reversed during the following growth period (between 21 and 40 days of age), with birds from enlarged broods growing at a faster rate than nestlings from reduced broods (Fig. 1). Finally, there was almost no increment in body mass during the third period (between 41 and 60 days of age) and, thus, the two brood size groups did not differ with respect to growth in this period (Fig. 1). Therefore, birds from enlarged broods had slow growth during the first period, accelerated growth during the second period and similar growth during the third period when compared to reduced-brood nestlings. To corroborate the idea that birds with fast early growth (during the 1–20 days of age) had slower growth afterwards and that nestlings with slow early growth had an accelerated growth afterwards, we found that growth rate during the 1–20 days of age period was negatively correlated with growth rate during the 21–40 days of age period (mixed model with growth rate between 21 and 40 days of age as dependent variable and growth rate between 1 and 20 days of age as independent variable: F 1,112 = 5·08, P = 0·026, parameter estimate ± SE: –0·131 ± 0·0578, R2 = –0·135).
Table 1. Repeated-measurements mixed model reporting the effect of period (between 1 and 20, 21 and 40, 41 and 60 days of age) and the experimental brood size on increment in body mass (g)
|Experimental brood size|| 2·50||1,135|| 0·116|
|Experimental brood size × Period||38·77||2,318||<0·0001|
|Individual|| 3·19|| 0·0007|
|Original brood|| 1·32|| 0·093|
|Foster brood|| 1·19|| 0·117|
Figure 1. Growth rate (body mass increase between days 1–20, 21–40 and 41–60 of age) of zebra finches reared in reduced (black bars) or enlarged broods (white bars). Least square means ± SE are reported.
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Although the difference in body mass between birds issued from enlarged and reduced broods clearly decreased with time, the two groups were still statistically different at the age of 60 days (F1,13·7 = 7·01, P = 0·020; 15·77 ± 0·27 g and 14·88 ± 0·28 g for birds in reduced and enlarged broods, respectively). However, when birds were recaptured about 3 months after their release into the aviary, body mass of brood reduced and brood enlarged birds was statistically indistinguishable (P = 0·401).
To test the effect of compensatory growth on red blood cell resistance to free radicals, we ran a model with experimental brood size as factor and the three growth rate periods as covariates. We found that only growth rate during the 21–40 days of age period was negatively correlated with red blood cell resistance to free radicals (Table 2 and Fig. 2). Interestingly, growth rate during the first age period (1–20 days) was not significantly correlated with red blood cell resistance to oxidative damage, suggesting that the difference in resistance to oxidative stress between brood size groups (i.e. Alonso Alvarez et al. 2006) was not due to the unfavourable conditions experienced during the first days of life, but more likely to the compensatory growth that took place when birds began to be independent from their parents. Thus, if the experimental brood size is maintained alone into the model, its effect is significant (P < 0·05). Nevertheless, if the increase in body mass during the 21–40 day period is added as a covariate, the effect of the experimental brood size loses its significance (P = 0.43). In fact, only body mass gain during the 21–40 day period remains as significant term after a backward step-wise procedure (note in Table 2). This supports the idea that the difference in oxidative stress resistance between brood size groups was indeed due to the effect of accelerated growth in some birds, mostly those in enlarged broods.
Table 2. Mixed model reporting the effect of experimental brood size and the three growth periods on the red blood cell resistance to free radicals assessed at the age of 60 days
|Experimental brood size||1·05||1,45·4||0·319|| || |
|Increment in body mass during the 1–20 day period||1·40||1,101||0·240||–0·302||0·255|
|Increment in body mass during the 21–40 day period||7·72||1,99·3||0·007||–1·120||0·403|
|Increment in body mass during the 41–60 day period||1·18||1,108||0·279||–0·531||0·488|
Figure 2. Relationship between red blood cell resistance to free radicals at 60 days of age and body mass gain during the 21–40 days of age in zebra finches reared in reduced or enlarged broods (filled and open dots, respectively). Red blood cell resistance to free radicals was assessed as the time needed to haemolyse 50% of erythrocytes exposed to a controlled free-radical attack. The linear adjust (estimated slope: –1·061 ± 0·341) was obtained from a mixed model on the whole of the sample taking into account the identity of original and foster nests as random factors (see text).
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Since birds from enlarged broods were those with the highest growth during the 21–40 day period (Figs 1 and 2), we might expect that the relationship between red blood cell resistance and growth should be mostly due to birds raised in enlarged broods. In agreement with this hypothesis, although the interaction between experimental brood size and growth rate was not statistically significant (F1,101 = 0·36, P = 0·550), we found that, when the correlation between red blood cell resistance to free radicals and growth rate was tested separately for the two groups of birds, it was statistically significant for enlarged-brood individuals (F1,64·1= 5·58, P = 0·021) but not for reduced-brood birds (F1,46·6 = 2·12, P = 0·152). In fact, the slope of the regression between mass gain and red blood cell resistances to free radicals was 36% steeper for birds of the enlarged brood compared to birds from reduced broods (estimated slopes ± SE: –1·29 ± 0·54 and –0·82 ± 0·56, respectively).
As mentioned above, using the residuals of a regression of body mass gain on the body mass at the beginning of each period (therefore controlling for among individual variation in previous body mass) provided the same results, with the growth rate during the 21–40 days period being the only significant predictor of red blood cell resistance to free radicals.
muscle growth and fat reserves
To understand the nature of the body mass gain, we analysed muscle and fat scores. Not surprisingly, both muscle and fat scores increased during the 21–40 day period, showing that birds continued to add resources to their muscular tissue and lipid stocks. However the rate of development was double for muscles (40d–21d scores; mean: ± SE: 0·41 ± 0·06) than for fat (0·23 ± 0·06). As for body mass, both muscle and fat scores stayed constant during the 41–60 day period (–0·05 ± 0·04 and 0·01 ± 0·04, respectively). Body mass gain between 21 and 40 days was positively correlated with the increase of muscle scores during that period (F3,72·8 = 4·50, P = 0·006) but not with the fat score increment (F3,72·8 = 1·49, P = 0·225). A similar result was also found for the 41–60 day period (muscle score: P = 0·035; fat score: P = 0·091). Birds from reduced broods showed higher fat scores from the beginning to the end of the sampling period (all P < 0·05). Muscle scores were higher in birds from reduced broods at 20 and 60 days of age (both P < 0·010), but not at 40 days of age (χ2 = 0·75, df = 1, P = 0·39). Chicks from enlarged broods showed a higher increase of muscle score during the 21–40 day period than chicks from reduced broods (means ± SE: +0·59 ± 0·08 and +0·16 ± 0·07, respectively; χ2 = 18·71, df = 1, P < 0·0001). The increase of fat score during the same period was slightly lower in birds from enlarged broods (means ± SE: +0·21 ± 0·08 and +0·26 ± 0·08, enlarged and reduced broods, respectively; χ2 = 3·93, df = 1, P = 0·047). There was no statistically significant difference in fat and muscle increments between brood size groups in the 41–60 day period (P > 0·05). Fat and muscle scores were not correlated with red blood cell resistance to free radicals (all P > 0·18).