proportion of birds in each age class that have laid eggs at any time previously
(Wyllie and Newton 1999). The oviduct of females that have yet to lay eggs
is thin and straight while that of females that have laid is wider and convoluted
(see diagrams in Wiltschi 1961; Wyllie and Newton 1999).
3.6.2Productivity from counts after the breeding season
Counts that yield ratios of young to adults can be used to estimate annual
productivity in some species. The validity of these counts depends upon whether
the survey method is equally likely to detect young and adults and whether young
and adults can be distinguished reliably. These conditions are met in species,
including cranes, swans, geese, and some galliforms, which are easy to observe
and have full-grown young with a distinctive plumage, which remain with their
parents. Surveys in autumn or winter then yield proportions of adults or pairs that
have reared different numbers of young, including the proportion of females
with no young, either because they were unsuccessful or did not attempt to breed.
An advantage of this method is that it is relatively easy and can therefore be applied
over large areas and sustained for many years. Good examples are the annual
estimation of the productivity of grey partridges by counts from a vehicle in August
that have continued without a break for more than 30 years on some study areas
(Potts and Aebischer 1995) and series of age ratio estimates for holarctic geese and
swans in their winter quarters, for example, the age ratio estimates for brent geese
Branta berniclaextend over more than 50 years.
For most birds, the estimation of productivity is more difficult. In some
species, young and adults can be distinguished by direct field observations, but
families do not stay together. In such cases, differential movements of adults and
young may render counts unreliable as an absolute measure, or even an index, of
productivity. For example, radio-tracking showed that young Marbled Murrelets
Brachyramphus marmoratuswere more likely than adults to move away from
coastal counting areas after the breeding season (Lougheed et al. 2002a). The
magnitude of this differential movement might vary among study areas and years
and invalidate comparisons.
3.6.3Productivity from captures after the breeding season
Relative numbers of young and adult birds caught in nets and traps soon after
the breeding season may be used to assess productivity for species that are
difficult to observe or have age classes that can only be distinguished in the hand.
This is likely to give an index of annual productivity, rather than an absolute
measure because of age differences in susceptibility to trapping, habitat selection,
and dispersal rate. If present, these sources of bias might be consistent across
74 |Breeding biology