Presentation on theme: "Brd Parasitism One of the rarest of reproductive strategies confined almost entirely to birds (with some fish as an exception) despite the fact that throughout."— Presentation transcript:
Brd 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. 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 females 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?? Its 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 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 216 1 – bay-winged cowbird, non-parasitic, but uses abandoned nests 7 71 176 screaming Shiny bronzed Brown-headed giant Nest-building relatives
Greater Coucal Common hawk cuckoo Plaintive Cuckoo ~53 spp of Old World Cuckoos Often specific, but may parasitize a diverse set of minor hosts
Indigobird Paradise whydah Pin-tailed whydah 16 spp of African Vidua finches Very 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 cont 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 nestlings palate and tongue (2)enlarged, light-reflecting and tubercular structures along edge of mandible (3)host juvenile plumage (4)virtually all of hosts major vocalizations mimic by adult male parasites (5)nestling begging vocalization and begging posture
Pin-tailed Whydah Common Waxbill Nestl. Juv.Adult Firefinch indigobird Purple grenadier Straw-tailed whydah Viduine finches and their Estrildine hosts. There are 125 spp of Estrildines each with unique mouth markings
Mimicry cont (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 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 distraction Fast egg-laying Cryptic plumage Nest-guarding Cryptic nest Restricted 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 0 20 40 60 80 100 CBC routes with cowbirds (%) 19001920194019801960 Increase in cowbird numbers and distribution In the 1900s
The plight of Kirtlands 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