linear dimension relative to the average value from a fitted growth curve for the
species gives a good indication of growth relative to the population mean.
In species in which bone and feather growth are less affected by food shortage
than the rate of increase in body weight, it may be possible to calculate an index
of condition for chicks of unknown age by expressing weight relative to some
function of linear dimensions. More details of condition indices can be found
in Chapter 4.
3.9 Proximate and ultimate causes of breeding failure
It is important to recognize that observational studies lead to assessments
about the relative importance of proximate factors influencing breeding success
and the causes of failure. However, there may be other ultimate factors that affect
breeding success indirectly. Knowledge of these ultimate factors may be useful not
only in improving understanding of population processes, but also in identifying
factors that may be susceptible to management for conservation purposes.
An example is the finding that Cirl Bunting Emberiza cirlusnestlings in a threat-
ened population frequently succumbed to predation and starvation (Evans et al.
1997). Analysis of the weight gain rates of nestlings showed that young in broods
that were taken by predators or starved both had low growth rates and that
broods that survived grew considerably faster. This suggested that habitat man-
agement to improve the food supply might reduce both causes of nestling loss.
3.10 Value of experiments to disentangle ultimate and
proximate causes of breeding failure
Although the relative roles of ultimate and proximate causes may be indicated
by observational studies of breeding, field experiments will often provide more
robust evidence. Experimental manipulations of many aspects of bird breeding
are feasible and include nest site enhancement or removal, supplementary feeding
and control of predation. A detailed review is beyond the scope of this chapter and
useful examples of experiments can be found in Newton (1994), Newton (1998,
chapter 7), and Tapper et al. (1996). However, a recent development in experi-
ments on bird breeding biology worthy of special mention is the use of mani-
pulations of the energy requirements of parent birds and nestlings. Yom-Tov
and Wright (1993) heated nest boxes of blue tits during the egg-laying period
and found that interruptions of egg-laying during cold weather were reduced.
Incubation requires a considerable amount of energy from parent birds and this
can be reduced experimentally by slowing the rate of cooling of the eggs using
Ultimate and proximate causes of breeding failure| 79