Transcript Slide 1
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