Migratory birds often switch their

Migratory birds often switch their diets seasonally.
For example, many waterfowl switch from
high carbohydrate foods Že.g. seeds. to high protein
foods Že.g. aquatic insects, new plant tissue.
when preparing for breeding ŽKrapu and Rei-
necke, 1992.. Also, many insectivorous songbirds
switch to feeding primarily on fruits during migration
ŽEvans, 1966; Herrera, 1984; Izhaki and
Safriel, 1989; Bairlein, 1990, 1991; Bairlein and
Gwinner, 1994; Biebach, 1996.. This dietary switch
from insects to fruits may conserve energy because
fruits that are abundant are less energetically
expensive to obtain compared to insects.
Fruits may be nutritionally adequate if only fat
reserves must be replenished. However, fruits may
be inadequate if birds must replenish both fat and
protein reserves during migration because, in
general, fruits contain relatively little protein ŽBi-
ebach, 1996.. Dramatic changes in dietary substrate
from, for example, protein-rich insects to
carbohydrate-rich fruits, offer significant physiological
challenges for birds ŽAfik and Karasov,
1995; Karasov, 1996.. Herein we review how
changes in diet quality affect digestive structure
and function in migratory birds, and how it may
influence the tempo of their migration.
2. Phenotypic plasticity and flexibility in
physiological systems
Physiological adaptation can be deduced from
comparative interspecific analyses of traits Že.g.
organ size and function, nutrient transport rates
and metabolic rate. with appropriate control for
phylogeny. Accordingly, a central theme of evolutionary
physiology involves conducting comparative
studies of physiological traits in vertebrate
taxa with different life styles ŽFeder et al., 1987;
Wainwright and Reilly, 1994.. However, many
such physiological traits exhibit considerable variability
in ecological time both within and among
individuals which may make it difficult to detect
important adapted patterns. The form of phenotypic
variation that involves a single genotype
producing different phenotypes in response to
variation in some environmental variable is called
‘phenotypic plasticity’ ŽTravis, 1994; Piersma and
Lindstrom, 1997.. Rapid reversible changes in
body composition, organ size, and digestive
processes provide examples of flexible norms of
reaction ŽStearns, 1989; Travis, 1994. or ‘phenotypic
flexibility’ in that they may represent flexible
responses to changes in the environment ŽPiersma
and Lindstrom, 1997.. Such phenotypic flexibility
in physiological traits may itself be a critical component
of the adaptive repertoire of animals that
may influence diet diversity, niche width, feeding
rate, and thus the acquisition of energy and essential
nutrients ŽKarasov, 1996; Kersten and
Visser, 1996; Pigliucci, 1996; McWilliams et al.,
1997; Piersma and Lindstrom, 1997..
Understanding the patterns and consequences
of phenotypic plasticity and flexibility has important
implications for animal ecology. Defining
these implications requires mechanistically linking
the study of the trait in its various forms with
some ecologically relevant performance criteria
ŽArnold, 1983; Wainwright and Reilly, 1994.. Ecomorphologists
have established a tradition of such
studies although their usual focus is on comparing
the average response of many individuals,
thus viewing variation in response or phenotypic
plasticity as a nuisance to be avoided or at least
for which control must be exercised. One of the
central themes in this paper is that both body
composition and digestive features of migratory
songbirds are modulated in response to environmental
change and this phenotypic flexibility has
important ecological consequences for birds during
migration.
3. Food intake and digestive performance
The energetic gains that are realized by a bird
when it eats more depend on interactions between
food intake rates and digestive efficiency ŽKara-
sov, 1996.. In the absence of extensive spare
digestive capacity, if the absorptive surface of the
gut or its capacity for absorption does not change
when a bird eats more, then the increased flow of
digesta may cause digestive efficiency to decline
and thereby directly discount the potential energetic
gains provided by hyperphagia. Alternatively,
if gut size increases with food intake, then
S.R. McWilliams,W.H. Karaso
Comparati
e Biochemistry and Physiology Part A 128 (2001) 579
593 581
digestive efficiency may not change when a bird
eats more. If tissue-specific digestive enzyme rate
or nutrient transport activity increases with food
intake Žwith no change in gut size., then digestive
efficiency may also not change when a bird eats
more. Below we discuss the effects of long-term
and short-term increases in feeding rates on digestive
performance in birds.
3.1. Effects of long-term increases in food intake on
digesti
e performance
Many studies of birds report increased surface
area and volume of the gut with long-term increases
in food intake ŽSavory and Gentle,
1976a,b; Savory, 1986; Dykstra and