Indirect, Physiological Assessment of Reproductive State and Breeding Chronology in Free-Living Birds: An Example in the Marbled Murrelet (Brachyramphus marmoratus)

1. An indirect, physiological method to assess reproductive state in individuals of unknown status is described. The plasma levels of two main yolk precursors, vitellogenin (VTG) and very-low density lipoproteins (VLDL), are focused on as indices of egg production, for the characterization of fecund females. 2. Data for a species where breeding chronology could be directly assessed, at the population level (Cassin's Auklet, Ptychoramphus aleuticus), confirmed the validity of this approach: plasma VTG levels were highest during the defined egg-laying period, and the highest proportion of females were defined as `egg-producing' in this period. 3. Analysis of samples for Marbled Murrelets (Brachyramphus marmoratus) caught off-nest (i.e. where all individuals were of unknown status), clearly identified a putative egg-laying phase, with a single, protracted laying period (cf. multiple-broodiness). 4. Analysis of body mass confirmed our characterization of `egg-producing' females: birds with elevated plasma VTG were on average 40 g heavier than other females, equivalent to the mass of the single egg (36-41 g). 5. Indirect, physiological assessment of reproductive state provided valuable information on the breeding biology of Marbled Murrelets which would have been difficult to obtain in any other way (e.g. proportion of fecund females, breeding phenology, single vs multiple-clutch breeding pattern). Despite some limitations, this technique should be applicable to any oviparous vertebrate population where essential information on breeding biology cannot be obtained by more traditional methods.