## 4 Discussion
### Rearing Period
#### 4.1.3 Influence of Stocking Density  
There was a significant effect of the stocking density on plumage damage in the first rearing period. The model for both rearing periods confirmed the significant effect of the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Factor** & **Significance (\(p\))** & **Partial Eta-Squared** \\ \hline Free range & 0.089 & 0.144 \\ Perch/laying hem (cm) & 0.165 & 0.099 \\ Enrichment & 0.450 & 0.030 \\ Mixed flock & 0.607 & 0.014 \\ Litter depth & 0.791 & 0.004 \\ \hline LV3 & & \\ \hline Corrected model & 0.001 & 0.722 \\ Constant term & 0.016 & 0.283 \\
**Rearing** & **0.017 \({}^{*}\)** & **0.362** \\
**Litter quality** & **0.015 \({}^{*}\)** & **0.289** \\
**Free range** & **0.020 \({}^{*}\)** & **0.266** \\ Perch/laying hem (cm) & 0.332 & 0.052 \\ Enrichment & 0.541 & 0.021 \\ Litter depth & 0.564 & 0.019 \\ Mixed flock & 0.901 & 0.001 \\ \hline \hline \end{tabular}
\end{table}
Table 7: _Cont._stocking density for RV2. The factor explained 40% of the variability of the dependent variable 'plumage damage in the rearing period'. Presumably, this factor was significant only at RV2 because the birds had grown, and took up more space than at RV1, approximately eight weeks earlier. Our result is in line with the findings of several other studies, which found high stocking densities during the first four weeks of rearing to be a risk factor for the development of SFP [26,34]. Owing to a lack of binding regulations in Germany for the rearing of laying hens, some rearing flocks had very high stocking densities. Fifteen flocks exceeded the current recommendations for stocking density (18 pullets per square meter of usable area from the 21 st day of life) [71]. A scientific approach to calculating maximum stocking densities suggests no more than 9-15 layer pullets per square meter of usable area [72], which was exceeded by 21 out of 30 flocks in our study.  
According to [PERSON] [37], overcrowding--even if pullets seem to be able to adapt to the situation in the short term--may have long-term aversive effects on behavioral and physiological parameters. Hence, it seems possible that stress due to overcrowding in the rearing period is the basis for the development of SFP later in life. Typically, problems with SFP start at the peak of the laying period, around the 30 th week of life--which is a physically stressful phase for laying hens--and increase during the laying period [73,74]. Other studies on commercial pullet farms found significantly less SFP if the stocking density was approximately 18 pullets per square meter of usable area, compared with a 20% higher stocking density [32], although no significant difference in plumage condition was detected in the same flocks [36]. The authors presumed this may have been due to a not-detailed-enough scoring scheme. However, pullets--like any other animal--seek to maintain certain social distances to conspecifics. If this is not possible, owing to high stocking densities, certain behaviors (e.g., foraging) are reduced or stopped entirely. The lack of foraging behavior may lead to misdirected behavior and pecking on the feathers of conspecifics and, in due course, to SFP [7,8,15].