Br dParasitism

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 thenorm.

Among Chordates it is only birds with externally exposed eggs and intense parental care for which there is an opportunity for reproductive exploitation

Tailorbird feeding a Plaintive Cuckoo

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

Basic Terminology

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)

Tradeoff between parental care and adult survivorship is a universal property of life

Why should brood parasitism evolve?

- Freedom from parental care and its inescapable costs

- Freedom from clutch/brood limitations

- Avoid putting all your eggs in one basket – literally!!!

survival rate

parent

offspring

parental effort

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

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 hatchedchicks

Predominantly African species thatparasitize woodpeckers, barbets,bee-eaters and kingfishers

Who are the Obligate Brood Parasites??

Cowbirds – recently evolved brood parasites show a pattern of break down in territoriality, nest building, and expanding host generalism

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1 – bay-winged cowbird, non-parasitic, but uses abandoned nests

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176

screaming

Shiny

bronzed

Brown-headed

giant

Nest-building relatives

Greater Coucal

Common hawk cuckoo

Plaintive Cuckoo

~53 spp of Old World CuckoosOften specific, but may parasitize a diverse set of minor hosts

Indigobird

Paradisewhydah

Pin-tailedwhydah

16 spp of African Vidua finchesVery host specific

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

Pin-tailed Whydah Common Waxbill

Nestl.

Juv.Adult

Firefinch

indigobirdPurple

grenadier

Straw-tailedwhydah

Viduine finches and their Estrildinehosts.

There are 125 spp of Estrildines each with unique mouth markings

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 recentlyencounter 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

Parasitized Red-eyed Vireo nest

(1) egg removal/chick removal and damage

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

Steel-blue Whydah and its Black-cheeked waxbill host

Hawk mimicry and host distractionFast egg-laying Cryptic plumage

Nest-guardingCryptic nestRestricted nest entrance

Refined mimicry and Refined discrimination

Historically nomadic following grazing buffalo herds that helped exposed seeds

Current distribution of the Brown-headed Cowbird

Parasitism intensities were likely historically low restricted to grassland bird and periphery forest populations

Today – buffalo substitutes (cows), waste grain, and fragmented landscapes have led to population increase, distribution expansion, and access to forest interiors

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CB

C r

oute

s w

i th c

owbi

rds

(%)

1900 1920 1940 19801960

Increase in cowbird numbers and distribution In the 1900s

The plight of Kirtland’s Warbler is the most renowned

Always a restricted species given its limited distribution requirements, added cowbird parasitism only exacerbated itsendangered 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?

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They are Host Generalistsand therefore lack negative feedback to their own populations

as one host species declines

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