of the cowbird relationships with these two hosts. COWBIRD lation with host losses to cowbirds or with parasite and host successes. The Auk, .. loss to cowbird removal observed in the report is tested in Table 6, and also. To test this hypothesis, we played audiovisual recordings showing the same Preferences female cowbirds show in acoustic CSD studies are .. the precise quantitative relationship between variation in CSD durations in. In this relationship, the brood parasite depends entirely on the host for . We tested whether parasitic cowbirds differ from nonparasitic species in some of the .
Therefore, we used data of females when possible. To minimize errors, we used data on the average growth rate and the asymptotic weight reached in the nest estimated using the methodology of Ricklefs If an author did not use this approximation, we used the original data to estimate both parameters.
Kattan suggested that the parasitic Molothrus bonariensis has accelerated its development by leaving the nest with a lower proportion of the adult weight. Therefore, we regressed the asymptotic weight reached in the nest to female body weight. We increased the number of species included in that analysis using asymptotic weight data as estimated by Ricklefs' methodology and those reported as weight of the nestlings before leaving the nest Appendix.
Authors that suggested adaptations to brood parasitism worked with different subspecies of cowbirds.
Specifically, in Molothrus bonariensis, seven subspecies have been described whose adult female weights vary from Therefore, we tried to include data of more than one subspecies of M. We applied the logarithmic transformation to all variables before the analysis. Phylogenetic hypothesis Parasitic cowbirds were included in one of the five natural groups within the Icteridae: The grackles and allies' phylogeny was based on the sequences of cytochrome-b and ND2 mitochondrial genes and was completely resolved with rather good support Johnson and Lanyon, As an estimation of the evolutionary divergence between taxa tip or node specieswe used the number of substitutions in both sequences with the character weights given by Johnson and Lanyon.
This information was not available in Johnson and Lanyon Therefore, to obtain branch length data, we had to access the sequences of both genes for all species in the GenBank, and then we repeated the author's analysis with PAUP 3. When performing our comparative analysis, we only replaced Dives warsaweski by the most known congener, D. Consequently, comparative data are not statistically independent and therefore violate one of the most basic assumptions of statistical procedures Felsenstein, ; Martins and Hansen, Phylogenetic independent contrasts PIC are calculated as differences in trait values between adjacent pairs of nodes or terminal taxa in a phylogenetic tree.
Because contrasts do not share the same branches of a tree, they are statistically independent samples of evolutionary change within a lineage. We did not use the PIC as originally stated Felsenstein,but used an extension of that method proposed by Garland and co-workers. In this extension, PIC are used to generate an allometric regression line with its prediction intervals including the Y intercept of the original data Garland and Ives, We generated the prediction interval for a nonparasitic grackle and allies that evolved in the place of the cowbirds.
If observed values of parasitic cowbirds differ from those predicted by the regression, the adaptations to brood parasitism may hold Garland and Ives, Otherwise, the existence of exaptations can be claimed.
The assumptions of PIC are 1 the topology of the phylogenetic tree is dichotomous, fully resolved, and accurate and 2 branch length is a good estimator of the expected variance in character evolution. Consequently, contrast values can be standardized to a mean 0 and a variance 1 through dividing them by the square root of the sum of the branches involved in the comparison Felsenstein, ; Garland et al. If properly standardized, contrasts can be used to perform basic statistics such as correlation and regression Felsenstein, ; Garland et al.
We performed the analysis assuming two evolutionary models: Under the BM model of evolution, changes in character states follow a random process with mean 0 and branch length as expected variance.
To guarantee that the PE analysis is not biased, the data set must include all species of the clade.
We tested whether parasitic cowbirds differ from nonparasitic species in some of the proposed life-history characters. We had to prune off the clade that includes all parasitic species from the tree before the analysis Figure 1 ; Garland and Ives, Under the BM evolutionary model, we used the original tree topology with branch lengths as shown in Figure 1.
If information on character states of a tip species was not available, we pruned it off again. Under the PE evolutionary model, we had to avoid the bias of not considering all speciation events, so we first set up all branch length of the original tree to 1. Then we pruned off all the taxa without character information. As we transformed branch length to 1 before pruning, distances between nodes and remaining tip species in the original tree were kept in the pruned tree.
With both BM and PE evolutionary models, we tested if the generated contrasts were correctly standardized. Similar to a simple linear regression, prediction intervals of PIC are generated considering which value of the dependent variable is expected under a particular value of the independent one Garland and Ives, ; Sokal and Rohlf, Moreover, PIC prediction intervals take into account the location of the species of interest and the length of the branch that attaches it to the tree Garland and Ives, It is expected that species would be more similar to closely than to distantly related species.
Moreover, the length of the branch leading to hypothetical species would be an estimator of its independent evolution Garland and Ives, To generate the prediction interval the original tree must be re-rooted in an internal node. With this re-rooting, the species in question must be adjacent to its sister taxa and to the remaining tree Garland and Ives, ; Garland et al.
If the sister taxa of parasitic cowbirds were reduced to one species, we choose to re-root the tree in the midpoint of that branch Garland et al. Generation of the prediction intervals was originally stated for only one taxon, and there are five species parasitic cowbirds.
Therefore, we generated the prediction intervals using an average of the branch lengths that attached each cowbird species to the tree.
Under a BM evolutionary model, the branch length we used was Results obtained with averages did not differ from other alternatives such as using the minimum or the maximum branch length of the cowbird's clade.
When we had applied a transformation to branch lengths, the branch of the cowbirds was transformed in the same way. The phylogenetic tree will look like it has a unique, hard i. It has been suggested that if a range of evolutionary models all lead to the same conclusions, those conclusions are strengthened Garland and Adolph, ; Garland et al. Therefore, we repeated all analysis considering the star phylogeny as an alternative. In practice, we performed simple linear regression with all variables using data of nonparasitic species to build the associated prediction intervals.
In all cases, both variables were continuous and subjected to error. However, as we performed the analysis mainly for a predictive purpose, we applied the model I of the regression Sokal and Rohlf, RESULTS Adjustment and significance of the three evolutionary models The correlations generated under BM and PE models, as well as under the simple regression of the star phylogeny, showed similar patterns of significance and adjustment Table 1.
Only the relationship of the average growth rate K and the asymptotic weight reached in the nest A under both the BM and the PE models were marginally significant. In a similar way, the adjustments with simple linear regression were significant except for the relationship of the average and minimum incubation period and egg weight, which were nonsignificant and marginally significant, respectively Table 1.
Contrasts and prediction intervals under a Brownian model Most contrasts were adequately standardized. Recent model simulations indicated that the strategy of exploiting host compensatory responses may be common, particularly in nontrophically transmitted parasite systems Dubois et al.
In many short-lived bird species, variation in the number of nestlings fledged from a successful nesting attempt may influence the likelihood of initiating another nesting attempt within the same season i.
Parental investment beyond the nesting stage i. Partial brood reduction i. By affecting the total number of fledglings produced, brood parasites could affect the frequency of double-brooding in host species capable of double-brooding. Host nestlings with less competitive ability than their parasitic nestmates often perish; and this competition for parental care likely continues during the fledgling stage, resulting in increased mortality of host fledglings Rasmussen and Sealy ; Peterson et al.
Hosts producing a reduced total number of fledglings from their first nesting attempt, regardless of parasitism status, may be more likely to double-brood i. An increase in host double-brooding i. Brood parasitism, alternatively, could reduce the likelihood of hosts making additional nesting attempts.
By definition brood parasitism is energetically costly to the host as parasitic nestlings generally demand increased amounts of resources relative to the host offspring Hoover and Reetz ; Mark and Rubenstein ; but see Canestrari et al. Parasitic nestlings typically weigh much more than host nestlings, often more than the combined weight of the entire host brood.
Here, we investigate the impact of brood parasitism by brown-headed cowbirds Molothrus ater on the probability of double-brooding in an individually marked population of a multiply brooding host species, the prothonotary warbler Protonotaria citrea. Using data collected from both parasitized and nonparasitized successful nests, we examine the effects of fecundity reduction and brood parasitism on the probability adult female warblers initiate an additional clutch within the same season.
We predict that 1 natural levels of fecundity reduction e. Specifically, we predicted that female warblers with parasitized nests would fledge the least offspring yet have a greater likelihood to double-brood.
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Similarly, we predicted that female warblers associated with experimentally parasitized nest-boxes would double-brood at a greater frequency when compared with females using nest-boxes where all effects of cowbirds were excluded.
We also used radio-telemetry to determine the fate of 26 cowbird offspring to account for potential effects of postfledging cowbird mortality on double-brooding in the host. Both female cowbirds Hahn et al.Animal partnerships - David Attenborough - BBC wildlife
We compared parasitism rates during the additional breeding attempts, using the parasitism status in the first brood as a predictor to identify nonrandom parasitism in the subsequent brood indicative of realized enhancement of transmission. The prothonotary warbler is a Neotropical migratory songbird that inhabits flooded bottomland forests and swamps throughout eastern North America Petit This species is territorial, socially monogamous Petitand as a secondary cavity nester, readily uses nest-boxes when provided.
Successful breeding pairs are highly territory faithful within a season, initiating additional clutches in the same or neighboring nest-boxes Hoover a. Typically double-brooded, prothonotary warblers lay 4—5 eggs during each nesting attempt Petit As a host, the prothonotary warbler appears to lack adaptive responses to brood parasitism Hoover band although able to raise a few warbler offspring with a cowbird nestling, parasitism events typically result in a reduction of host fecundity Hoover c.
Common throughout North America, brown-headed cowbirds are obligate brood parasites that have been successfully raised by approximately host species Lowther Telemetry Dufty ; Raim and genetic studies Hahn et al. In prothonotary warbler nests, cowbird nestlings increase the rate of food provisioning by adult warblers Hoover and Reetz and, weigh on average 2—3 times more than warbler nestlings Hoover c. Each year, approximately nest-boxes 1.
Nest-boxes were placed 1. To limit nest predation, a majority of the nest-boxes were attached to greased conduit poles. We monitored nest-boxes every 3—6 days throughout the breeding season and we recorded the number of eggs and nestlings of warblers and cowbirds present at each visit. Nest initiation dates i. We assumed nestlings fledged if they reached 10—11 days of age and the nest was empty and intact.
Additional evidence of fledging included the presence of trampled droppings in the nest, alarm calls from adults, and observations of appropriately aged fledglings in the territory. Adult warblers were captured and fitted with a unique color-band combination and a numbered aluminum band.