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 indivi duals of unknown status is described. The plasma levels of two main yolk pr ecursors, 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), confir med the validity of this approach: plasma VTG levels were highest during th e defined egg-laying period, and the highest proportion of females were def ined as 'egg-producing' in this period. 3. Analysis of samples for Marbled Murrelets (Brachyramphus marmoratus) cau ght off-nest (i.e. where all individuals were of unknown status), clearly i dentified a putative egg-laying phase, with a single, protracted laying per iod (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 o ther females, equivalent to the mass of the single egg (36-41 g). 5. Indirect, physiological assessment of reproductive state provided valuab le information on the breeding biology of Marbled Murrelets which would hav e been difficult to obtain in any other way (e.g. proportion of fecund fema les, breeding phenology, single vs multiple-clutch breeding pattern). Despi te some limitations, this technique should be applicable to any oviparous v ertebrate population where essential information on breeding biology cannot be obtained by more traditional methods.