Transcript Parental Care III: Clutch Size

Parental Care III: Clutch Size
JodyLee Estrada Duek, Ph.D.
With assistance from Dr. Gary Ritchison
http://people.eku.edu/ritchisong/avianreproduction2.html
Clutch size assumed to be affected by…
• short-term constraints (proximate factors, e.g.
energy)
• long-term considerations (ultimate factors, e.g. life
span)
Survival vs fecundity
• The optimization of the trade-off between annual adult
survival and annual fecundity.
• The bounded area within the curve represents the set of
possible phenotypes, among which those potentially having
maximum fitness are represented on the periphery.
• The adaptive function is a line running through combinations
of fecundity and survival having equal fitness and just tangent
to the fitness set.
• The tangent point is the phenotype that maximizes
evolutionary fitness.
• The slope of the line (−1/T) is the negative value of the
inverse of the average age (T) of a female at the birth of her
offspring.
• Thus, slope is related to the negative of the adult mortality
rate (From: Ricklefs 2000).
Clutch Size Factors 1: Age
• clutch sizes vary considerably among species (1 - 18
or more) but are less variable within species
• Things that influence clutch size within a species
include:
– Age - older females typically have larger clutches
Source: http://alaska.fws.gov/
The relation between hatching success and breeding age in Leach's StormPetrels on Kent Island, New Brunswick, Canada, 1962–1995 (N = 18,347).
Age-specific reproductive success
• has been demonstrated in many species.
• Three hypotheses have been raised to explain this general phenomenon:
the experience hypothesis based on age-specific reproductive experience,
the effort hypothesis based on age-specific reproductive effort, and the
selection hypothesis based on progressive disappearance of phenotypes
due to variation in individual productivity and survival.
• Mauck et al. (2004) used data from a long-term study of Leach's stormpetrels (Oceanodroma leucorhoa) to test mutually exclusive predictions
about the relationship between early breeding success and longevity.
• There should be
• no correlation between early breeding success and longevity under the
experience hypothesis,
• negative correlation under the effort hypothesis
• positive correlation under the selection hypothesis.
• found a significant (P < 0.0001) positive relationship between success in first
two breeding attempts and longevity in a population of long-lived seabirds,
suggesting low-productivity parents less likely to survive early breeding.
• strongest support to date for the selection hypothesis.
Clutch Size Factors 2: Location & Resources
– Season - in temperate areas, clutches typically smaller
later in the season
– Food availability - clutches typically larger with increased
availability of food
– Latitude - clutch sizes typically larger at higher latitudes
Eastern Bluebird clutch size
• Predicted from a model based on 945 clutches; values for the year 1997 at 80° W
and at 30° N and 45° N.
• Clutch sizes farther south are small early in the season, increase to a midseason
peak, and become smaller again late in the season.
• Clutches farther north decline throughout the season from an initial high.
• Julian lay date expresses date as days since January 1 (From: Dhondt et al. 2000).
Clutch size evolution in excavating birds (next slide)
• Potential pathways, showing causal links left to right.
• Arrows within the boxes indicate whether factor increases or
decreases in response to causal agent preceding it; e.g., increasing
food stability causes increasing adult survival.
• The figure encapsulates the following hypotheses:
–
–
–
–
(H1) stability of food resources,
(H2) nest site limitation,
(H3) energy costs of excavation,
(H4) predation risk on the nest.
• Other factors that affect adult survival independent of excavation
ability may also influence clutch size.
Clutch size
evolution in
excavating birds
Wiebe et al. (2006)
Clutch sizes of all cavity-excavating birds
• two major competing hypotheses for variation in clutch size among cavity-nesting
species.
• nest site limitation hypothesis postulates that nesting opportunities are more
limited for weak excavators, which consequently invest more in each breeding
attempt by laying larger clutches.
• clutch size may be determined by diet; the clutch sizes of strong excavators may be
smaller because they are able to specialize on a more seasonally stable prey.
• Wiebe et al. (2006) built a conceptual model that integrated hypotheses for
interspecific variation in clutch size and tested it with comparative data on lifehistory traits of woodpeckers (Picidae) and nuthatches (Sittidae).
• diet explained more variation in clutch size among species than did propensity to
excavate.
• Migratory status was positively associated with clutch size, but was difficult to
distinguish from diet because resident species consumed more bark beetles (a prey
available in winter) and had smaller clutches than migratory species.
• The literature suggests that cavities are not limited in natural, old-growth forests.
• Although Wiebe et al.'s (2006) data do not rule out nest site limitation, the authors
concluded that annual stability of food resources has a larger impact on the
evolution of clutch sizes in excavators than does limitation of nest sites.
http://www.birdquest.co.uk/HolidaysbyRegion.cfm?Holiday=790
Syrian Woodpecker
• (Dendrocopos syriacus)
http://www.youtube.com/watch?v=unfOqXdVk58
Evolution of clutch sizes
• Possible factors involved:
• Lack's Food Limitation Hypothesis - clutch size is
adjusted by natural selection to the maximum number
of nestlings the parents can feed
– Problems with Lack's hypothesis:
• commonest clutch size in a population is often smaller than what
appears to be most productive
• some species can raise additional young when provided
experimentally
• The Trade-off Hypothesis
– also called the resource allocation hypothesis
– current reproductive effort should be influenced by longterm reproductive interests, i.e., large clutches & broods may
require so much investment (and risk) that adult survival
(and, therefore, future reproduction) may be affected
Source:
http://www.bbc.co.uk/nature/birds/birdpages/35.html
The cost of egg production
• increased egg production reduces future fitness in gulls –
• reproductive costs are essential to understand reproductive effort.
• effects of increased chick-rearing effort on survival and fecundity have been well
studied, costs associated with increased egg-production effort have not
• Nager et al. (2001) experimentally increased the egg-production effort
of individually-marked Lesser Black-backed Gulls (Larus fuscus) and followed
breeding performance the next year.
• In the following season, females, but not males, were less likely to be resighted and
those that did return were less likely to produce a clutch
• It is unclear whether local return rate represents
• differential survival
• differences in breeding propensity
• differences in dispersal between experimental and control
• experimental females that did breed invested less in egg production
• evidence that there is an inter-brood trade-off between current egg-production
effort and future fitness in birds.
Predation risk and clutch size 1
• Clutch size reduction under high risk of nest predation hypothesized to be adaptive
o First, when nest predation increases with clutch size, smaller broods will shorten the
period when the nest is susceptible to nest predators and reduce the number of nest
visits that could attract the attention of predators.
o Second, if parental survival declines with clutch size, then a reduction in clutch size will
improve parental survival and future reproduction, spreading the risk of nest predation
between broods and increasing lifetime reproductive success.
• Despite these predictions being consistent with life-history theory, there is little
experimental support for nest predation affecting clutch size variation in birds.
• theoretical and experimental studies on other taxa provide evidence for predatorinduced life-history shifts through phenotypic plasticity.
• Past work on bird clutch sizes has mainly involved intra- and interspecific
comparative analyses, which provides insights into factors associated with clutch
size variation, but does not necessarily infer a causal relationship.
(Photo source: http://www.animalpicturesarchive.com).
Predation risk and clutch size 2
• Eggers et al. (2006) manipulated perceived risk of nest predation in
Siberian Jays (Perisoreus infaustus) using playback of a mixture of
calls by corvid nest predators near nest sites.
• In response to this acoustic cue simulating increased risk of nest
predation, the jays chose
o a nest site offering more protective covering and
o reduced clutch size.
• This is the first experimental demonstration of clutch size adjustment
and nest site selection as a result of phenotypic plasticity in an open
nesting passerine reflecting a facultative response to the perceived
risk of nest predation
Egg Predator
• Snake eating an egg
http://www.metacafe.com/watch/87916/snake_eating_an_egg/
Nest predator risk and reproductive strategies
• It has been assumed that nest predation plays a minor role in
reproductive strategies.
• untested by experiments to alter risk of nest predation, despite
nest predation being a major source of reproductive failure.
• Fontaine and Martin (2006) examined whether parents can assess
reduced nest predation risk and alter reproductive strategies.
• They experimentally reduced nest predation risk by removing
potential predators from study plots and showed that parents in
safer environments increased investment in young through
increased egg size, clutch mass, and the rate they fed nestlings.
• Parents also increased investment in female condition by
increasing the rates that males fed incubating females at the nest,
and decreasing the time that females spent incubating.
• These results demonstrate that birds can assess nest predation risk
at large and that nest predation plays a key role in the expression
of avian reproductive strategies
Clutch Size
• The Predation Hypothesis - High predation rates could
select for smaller clutches because:
– it takes longer to lay a large clutch than a small one (so eggs
& young in a large clutch are vulnerable to nest predators
longer)
– larger broods are noisier and require more nest visits by
adults, both of which might attract predators
– where risk of predation is great, it may be better to risk
fewer eggs and young at a time (assuming there will be
additional breeding opportunities)
• The Seasonality Hypothesis
– clutch sizes in some birds may be influenced by variation in
the seasonal availability of resources
What factors affect clutch size? 1
• In tropical regions, female birds
tend to have fewer young per nest
• debate over whether clutch size limited by food, by
predators, or both.
• Alexander Skutch suggested that predators force
parents to limit rate at which they visit nest,
resulting in evolution of smaller families.
• parents who fly off nest to find food may attract the
attention of predators.
• Skutch's hypothesis had never been tested.
What factors affect clutch size? 2
• Thomas Martin et al (2000) monitored 1,331 nests in subtropical Argentina &
7,284 nests in Arizona.
• As expected, clutches smaller in Argentina (2.58 eggs/nest) than Arizona (4.61
eggs/nest).
• analyses in each region seemed to support the Skutch theory because clutch size
was smaller for species with higher predation rates, where researchers also saw
fewer but bulkier meal deliveries.
• Nest predation constrained food delivery and clutch size, as expected.
• When researchers compared overall results from Argentina with those from
Arizona, however, some of their data didn't fit well with the Skutch theory.
• In Argentina
• predators were far less troublesome, and
• females brought food to their young faster, yet
• they had smaller clutch sizes than the Arizona birds.
What factors affect clutch size? 3
• According to the Skutch hypothesis, researchers should have seen
•
•
•
higher predation rates and
smaller, less frequent feedings
among the South American bird families.
• In Argentina, monitoring clutches with about half the number of nestlings that they
saw in Arizona, and yet, parents were bringing food at higher rates
• Clearly, smaller clutch sizes in southern areas can't be explained solely by food
delivery or predation rates
• WHAT OTHER FACTORS COULD BE INVOLVED?
• Martin & his colleagues recommend building on the Skutch theory by assessing
parent mortality rates in northern and southern climates.
• Birds that must endure cold winters or long migrations may need larger clutches to
maximize their fitness (Martin et al. 2000).
Seasonality, predation, food supply, and pathogens
(diagram on next slide)
• suggested as factors in small clutch sizes of tropical passerines
• questions remain concerning the relative importance of these
factors and how they might interact (a).
• Ricklefs and Wikelski (2002) outline a possible solution (b).
• A variety of environmental factors could act on separate
physiological systems, each of which could influence
the number of offspring.
• For example, pathogens influence the immune system, food
impacts the metabolic system, and environmental
unpredictability might interact with the endocrine system.
• Because of internal physiological tradeoffs between systems,
and because of system constraints, ultimate outcome might
be the same: a slow pace of life in tropical passerines, as
indicated by a uniformly small clutch size and long life span.