Download Brood parasitism

Br
d
Parasitism
One of the rarest of reproductive strategies
confined almost entirely to birds (with some
fish as an exception) despite the fact that
throughout the animal kingdom, a parasitic
lifestyle (internal and external) is probably the
norm.
Tailorbird feeding a Plaintive Cuckoo
Among Chordates it is only birds with externally
exposed eggs and intense parental care for
which there is an opportunity for reproductive
exploitation
Basic Terminology
Brood Parasitism – the surreptitious addition of eggs to another female’s nest,
whether of the same (intraspecific) or different (interspecific) species
Parasite – Benefits through increase # eggs w/o paying the cost of parental care
Host – Raises parasite young at a cost to its own fitness
Obligate brood parasites – never build their own nest or raise young (< 100 spp)
Facultative intraspecific brood parasitism or egg-dumping – deposition of eggs
into a common nest by several females
Coevolutionary Arms Race - “…a trait in one species has evolved in response to
a trait of another species, which trait was itself evolved in response to the first
species” (Futuyma and Slatkin 1983)
Why should brood parasitism evolve?
Tradeoff between parental care and adult survivorship is a universal property of life
offspring
survival
rate
parent
parental effort
- Freedom from parental care and its inescapable costs
- Freedom from clutch/brood limitations
- Avoid putting all your eggs in one basket – literally!!!
Egg-dumping Typically occurs in precocial spp, especially waterfowl
(63 of 79 spp) where rates may average ~20% (marsh-nesting) to 35%
(cavity-nesting).
Also in weavers (including House sparrow), starling, and marsh-nesting blackbirds
Wood Duck – the Egg-dumping
champion 23-95% nests have
been dumped in
Who are the Obligate Brood Parasites??
Indicatoridae – 18 spp of honeyguides
Old parasites?? It’s the only family
that is exclusively brood parasitic
and some species have structural
modifications for killing newly hatched
chicks
Predominantly African species that
parasitize woodpeckers, barbets,
bee-eaters and kingfishers
176
7
71
216
1 – bay-winged cowbird,
non-parasitic, but uses
abandoned nests
Nest-building
relatives
Brown-headed
Shiny
bronzed
giant
screaming
Cowbirds – recently evolved brood parasites show a pattern of break down in
territoriality, nest building, and expanding host generalism
~53 spp of Old World Cuckoos
Greater
Coucal
Often specific, but may parasitize a
diverse set of minor hosts
Common hawk cuckoo
Plaintive
Cuckoo
16 spp of African Vidua finches
Very host specific
Paradise
whydah
Pin-tailed
whydah
Indigobird
The Black-headed Duck –
the only obligate brood parasitic
waterfowl. South American.
Host adaptations and mimicry
(1) BPs choose smaller hosts (mean 60-70% adult parasite); not too small that
hosts hatch early or cannot feed larger chicks, not too large to be out competed
(2) Incubation period is on average 1-2 days shorter than hosts
(3) Raptor mimicry (plumage and threat displays) in several Cuckoos to intimidate
or distract host away from the nest
Mimicry con’t
Nestling mimicry is absent in Host-intolerant spp (Cuckoos) but in Viduine
finches it is little short of unbelievable....mimicking:
(1) interior mouth markings on nestling’s palate and tongue
(2) enlarged, light-reflecting and tubercular structures along edge of mandible
(3) host juvenile plumage
(4) virtually all of host’s major vocalizations mimic by adult male parasites
(5) nestling begging vocalization and begging posture
Adult
Juv.
Viduine finches and their Estrildine
hosts.
There are 125 spp of Estrildines
each with unique mouth markings
Nestl.
Pin-tailed Whydah
Common Waxbill
indigobird
Firefinch
Purple
grenadier
Straw-tailed
whydah
Mimicry con’t
(1) Eggs matched for size – approximately.
Parasite eggs tend to be somewhat larger
and more spherical to hold more potential
energy, increase difficulty in picking up, and
deflect potential blows of the beak
(2) thicker egg shells
(3) Egg color and pattern
(Cuckoos and in the more host specific cowbirds)
Host-race formation in Cuckoos??
Whereas the Brown-headed Cowbird, a recently
encounter parasite species for many hosts,
does not use cryptic egg coloration
Parasitized Red-eyed Vireo nest
Parasitized Wood Thrush nest
The Coevolutionary race – Part I – the Cost of Parasitism
(1) egg removal/chick removal
and damage
Parasitized Red-eyed Vireo nest
Common cuckoo
The Coevolutionary race – Part I – the Cost of Parasitism
(2) Low hatching and fledging success
due to abnormally high clutch/brood size
Such multiple parasitism is common in
the Brown-headed Cowbird, perhaps due
to nest-site limitation?
The Coevolutionary race – Part I – the Cost of Parasitism
(3) chick smothering or otherwise
“sibling” competition
The Coevolutionary race – Part I – the Cost of Parasitism
(4) “Spite” – increased nest predation on non-parasitized nests
The Coevolutionary race – Part II – the Response
(1) Hosts can be acceptors or rejecters – actually a continuum, and
a function of time of sympatry
Egg removal – e.g., catbird
Egg smashing – e.g., orioles
Nest abandonment – many passerines in NA
Build a second nest on top the original – Yellow warbler
(2) Reject nestlings
(3) Act aggressively towards parasites – many passerines in NA
The Coevolutionary race – Part III
Further Escalation
Hawk mimicry and host distraction
Fast egg-laying
Cryptic plumage
Nest-guarding
Cryptic nest
Restricted nest entrance
Refined mimicry and
Refined discrimination
Steel-blue Whydah and its
Black-cheeked waxbill host
Historically nomadic following grazing buffalo herds that
helped exposed seeds
Parasitism intensities were likely historically
low restricted to grassland bird and periphery
forest populations
Current distribution of the Brown-headed Cowbird
Today – buffalo substitutes (cows), waste
grain, and fragmented landscapes have
led to population increase, distribution
expansion, and access to forest interiors
100
80
Increase in cowbird numbers
and distribution In the 1900s
60
40
20
0
1900 1920 1940 1960 1980
The plight of Kirtland’s Warbler is the most renowned
Always a restricted species given its
limited distribution requirements, added
cowbird parasitism only exacerbated its
endangered status until cowbird eradication
programs took effect in the 1970s
Why do Brown-headed Cowbirds represent significant threats to several songbird
populations whereas cuckoos and the remaining brood parasites do not?
X
X
They are Host Generalists
and therefore lack negative feedback to their own populations
as one host species declines
X
X