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THE WILSON JOURNAL OF ORNITHOLOGY* Vol 123, No. 1, March 2011 
FIG. 1. Effects of migratory condition on muscle 
membrane PC/PE ratio in captive White-throated Sparrows 
subjected to photoperiod manipulation. There is no 
significant difference in the PC/PE ratios between long 
day (migratory) and short day (non-migratory) sparrows. 
Sample sizes for each group are presented across the top. 
Data are means + SE. A single long day adult had a PC/PE 
ratio of 3.42. 
FIG. 2. Effects of season and age on the muscle 
membrane PC/PE ratio in free-living White-throated 
Sparrows. There was a significant interaction between 
season and age. ^Wintering adults had a higher PC/PE ratio 
than adults in other seasons (P < 0.05). Sample sizes for 
each group are presented across the top. No juveniles were 
caught in spring. Data are means + SE; samples with 
unknown age are not shown. Two more wintering sparrows 
of unidentified age had PC/PE ratios of 2.1 and 3.4, and two 
more fall sparrows of unidentified age had PC/PE ratios of 
1.41 and 1.17. 
RESULTS 
TLC-FID clearly resolved the phospholipid 
classes in the standard mix. The only measurable 
phospholipids in the muscle samples were PC and 
PE, we therefore concentrated our analyses on the 
ratio of PC to PE. The total PC concentration 
exceeded the total PE concentration in 89% of 
wild sparrows and 100% of captive sparrows. 
Age was not investigated as a factor influencing 
the PC/PE ratio in captive birds because only one 
of the captives was an adult; this bird had a 
notably high PC/PE ratio (3.42). The PC/PE ratio 
among captive sparrows did not vary by photo¬ 
period treatment (P = 0.34; Fig. 1). The PC/PE 
ratio varied significantly with season and age 
among free-living sparrows, but there was a 
significant interaction between age and season 
(P = 0.032). There was no significant effect of 
season within juveniles (.P = 0.48) but, within 
adults, wintering birds had higher PC/PE ratios 
than migrants (P < 0.05, Fig. 2). Body mass had 
no effect when included as a covariate in the 
analysis. 
DISCUSSION 
Common polar head groups include serine, 
inositol, choline, and ethanolamine, but only the 
latter two are found in high concentrations in 
skeletal muscle (Mitchell et al. 2007); our results 
are in accordance with this previous observation. 
PC has a relatively large head group in relation to 
its fatty acid moieties, and forms a cylindrical 
molecular shape. Conversely, PE has a relatively 
small head group in relation to its fatty acid 
moieties, which results in a conical molecular 
shape and destabilizes membranes, making them 
more fluid (Logue et al. 2000). We focused on the 
ratio of PC/PE because the balance of these two 
phospholipids is important in affecting membrane 
characteristics (Logue et al. 2000). 
Membrane class composition may change 
adaptively for many reasons, including regulation 
of membrane fluidity, integrity, and interactions 
with membrane-bound proteins (Hazel 1995, 
Nagahuedi et al. 2009). Our results from wild 
sparrows are consistent with an increase in muscle 
membrane fluidity during migratory periods, 
although this may have been offset by a decrease 
in the double bond index of the fatty acid 
composition (Klaiman et al. 2009). It is unclear, 
however, why the season effect was only observed 
in adults. 
Our results are not consistent with any 
endogenous change to phospholipid class compo- 
