What comes first, the zebra finch or the egg? Resource allocation during avian egg production

Energy is an essential resource that all living organisms must balance. In vertebrates, very-low density lipoprotein (VLDL) particles play a key role in the transport of energy-rich lipids. During avian egg production, small yolk-targeted VLDL (VLDLy) particles are produced that can pass through the various layers of the ovary and are less susceptible to general metabolism, thereby preserving them for use by the developing ovarian follicles. To investigate how reproductive status and environmental conditions influence the differential allocation of energy-rich lipids to self-maintenance versus egg production in passerine birds, changes in VLDL particle diameter were characterized in relation to the reproductive status and output of Zebra Finches (Taeniopygia guttutu) exposed to favorable and energetically-challenging conditions. While birds producing eggs in favorable conditions exhibited a higher proportion of circulating VLDLy particles than non-breeding females, variation in VLDLy levels was not related to reproductive performance. When the energetically-demanding processes of thermoregulation and egg production were combined, laying birds consumed more seed and decreased locomotor activity, but took longer to initiate egg laying and laid fewer eggs at a slower rate than under warmer conditions. Cold-acclimated, laying females exhibited a decrease in the proportion of circulating VLDLy particles, but this decrease was not related to changes in reproductive effort. While these results suggest that maintaining a certain proportion of circulating VLDLy may not be an important factor in determining reproductive output in Zebra Finches, they were based on indirect estimates of VLDLy abundance. To develop an assay to directly quantify circulating VLDLy in passerine birds, two polyclonal antibodies raised against an apolipoprotein component of chicken VLDLy (apoVLDL-11) were tested. Putative Zebra Finch apoVLDL-I1 was expressed at lower levels in laying Zebra Finches and was less-well recognized by the antisera compared to chicken apoVLDL-I1 from laying hens, suggesting that inter-specific differences in the expression level or the structure of apoVLDL-I1 may make quantification of Zebra Finch VLDLy with existing antibodies impossible. Overall, these data suggest that egg-laying birds are able to allocate sufficient energy to fuel self-maintenance and sustain reproduction by increasing energy intake and reallocating energy from other activities.