• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

An investigation of factors influencing nestling begging behavior in a generalist brood parasite

Dissertation
Author: James William Rivers
Abstract:
Begging is a dynamic interaction in which dependent young solicit resources (typically food) from their care-giving parents, and parents use begging displays to make decisions about resource distribution among young. Theoretical studies indicate that the degree of relatedness between nestmates and the level of short-term need (i.e., hunger) experienced by an individual can strongly influence begging intensity. Obligate avian brood parasitism occurs when a female (the "brood parasite") lays an egg into the nest of another species (the "host"), which then provides all parental care to the parasitic chick. Because brood parasites are provisioned by unrelated "foster parents" and typically compete against unrelated nestmates, they are expected to act more selfishly and exhibit exaggerated begging behavior relative to nonparasitic species, all else being equal. This study was designed to elucidate the proximate and ultimate factors that may influence begging behavior in a generalist brood parasite, the Brown-headed Cowbird ( Molothrus ater ). The first three experiments assessed the extent to which two proximate factors (i.e., short-term need, nestmate size) influence cowbird begging, host begging, and provisioning by host parents. Taken together, these experiments found that short-term need generally increased the frequency and intensity of begging by cowbird and host chicks in the presence and absence of parents, that cowbirds typically begged more intensively than host chicks, and that a cowbird in the nest influenced host chick begging but not host food provisioning. Two additional experiments focused on the ultimate question of whether the exaggerated begging of the cowbird was an adaptation for brood parasitism and found that cowbirds did not exhibit begging displays that were more intense than those of the nonparasitic, closely related Red-winged Blackbird ( Agelaius phoeniceus ) as predicted by theory. Importantly, this relationship was consistent across two markedly different host species, and additional data suggest that high relatedness of cowbirds at Konza Prairie may constrain begging behavior. The final, observational study found that cowbirds parasitized nearly all available hosts at Konza Prairie, focused the majority of their parasitism on the Dickcissel ( Spiza americana ) and Bell's Vireo (Vireo bellii ), and were consistent in their use of hosts over the breeding season.

xv TABLE OF CONTENTS

I. General Introduction..............................................................................................1

II. Nestmate size, but not short-term need, influences begging behavior of a generalist brood parasite........................................................................................9 A. Introduction.........................................................................................10 B. Methods.............................................................................................. 14 C. Results.................................................................................................22 D. Discussion...........................................................................................30

III. Parent-absent begging in the Brown-headed Cowbird ( Molothrus ater ): the role of short-term need and host size...........................................................................36 A. Introduction.........................................................................................37 B. Methods...............................................................................................40 C. Results.................................................................................................50 D. Discussion...........................................................................................57

IV. Brown-headed Cowbird chicks influen ce host begging behavior, but not parental provisioning, in hosts of three distinct sizes........................................................64 A. Introduction.........................................................................................65 B. Methods...............................................................................................69 C. Results.................................................................................................77

xvi D. Discussion...........................................................................................91

V. Relatedness constrains the begging display of an obligate avian brood parasite..98 A. Introduction.........................................................................................99 B. Methods.............................................................................................102 C. Results...............................................................................................112 D. Discussion.........................................................................................119

VI. An experimental investigation to assess the extent and consequences of exaggerated begging behavior by the Brown-headed Cowbird.........................126 A. Introduction.......................................................................................127 B. Methods.............................................................................................130 C. Results...............................................................................................139 D. Discussion.........................................................................................147

VII. Community-level patterns of host use by the Brown-headed Cowbird, a generalist brood parasite....................................................................................153 A. Introduction.......................................................................................154 B. Methods.............................................................................................157 C. Results...............................................................................................163 D. Discussion.........................................................................................170

VIII. References.......................................................................................................179

xvii LIST OF FIGURES AND TABLES

II. Figure 1. Mean (95% CI) latency to beg for cowbird (solid circles) and host nestlings (open circles) in nests of (A) small, (B) intermediate, and (C) large hosts.................................................................................................25 Figure 2. Mean (95% CI) time spent begging for cowbird (solid circles) and host nestlings (open circles) in nests of (A) small, (B) intermediate, and (C) large hosts..........................................................................................26 Figure 3. Mean (95% CI) maximum begging posture for cowbird (solid circles) and host nestlings (open circles) in nests of (A) small, (B) intermediate, and (C) large hosts...................................................................................27 Figure 4. Mean (SE) for (A) host provisioning rates and (B) proportion of food received by cowbirds over both feeding treatments combined in small, intermediate, and large hosts....................................................................28 Figure 5. Mean (SE) for (A) time spent begging per food visit, (B) volume of food obtained per 10 min, and (C) mean payoff for cowbirds over both feeding treatments combined in small, intermediate, and large hosts......29

III. Table 1. Summary of the apparent stimuli that coincided with the initiation of parent-absent begging in cowbird and host nestlings..............................53

xviii Figure 1. Mean (SE) proportion of parent-absent begging events during which cowbird (solid circles) and host nestlings (open circles) begged during deprivation and supplementation treatments in nests of the (a) Field Sparrow, (b) Red-winged Blackbird, and (c) Brown Thrasher................54 Figure 2. Mean (SE) proportion of time spent begging during parent-absent periods for cowbird (solid circles) and host nestlings (open circles) during deprivation and supplementation treatments in nests of the (a) Field Sparrow, (b) Red-winged Blackbird, and (c) Brown Thrasher................55 Figure 3. (a) Mean (SE) begging posture, (b) mean (SE) time spent begging during parent-absent begging events, and (c) mean (SE) percent of events during which cowbirds (solid circles) and host nestlings (open circles) called in nests of the Field Sparrow (n=10 nests), Red-winged Blackbird (n=16 nests), and Brown Thrasher (n=13 nests)......................................56

IV. Table 1. Full model for repeated-measures analysis for effects of host size (sparrow, blackbird, or thrasher), presence or absence of a cowbird chick in the brood, and feeding treatment (deprivation or supplementation) on the mass-corrected hourly provisioning rate............................................79 Figure 1. Mean (SE) provisioning rate to (A) sparrow, (B) blackbird, and (C) thrasher nests over two feeding treatments and relative to whether a brood contained a cowbird and a host chick (parasitized) or two host chicks (unparasitized)..............................................................................80

xix Table 2. Full model for repeated-measures analysis for effects of host size (sparrow, blackbird, or thrasher), presence or absence of a cowbird chick in the brood, and feeding treatment (deprivation or supplementation) on the proportion of food obtained by first chick in the brood.....................81 Figure 2. Mean (SE) proportion of food obtained by first chick in the nest of the (A) sparrow, (B) blackbird, and (C) thrasher relative to whether a brood contained a cowbird and a host chick or two host chicks..............82 Table 3. Full model for repeated-measures analysis for effects of host size (sparrow, blackbird, or thrasher), presence or absence of a cowbird chick in the brood, and feeding treatment (deprivation or supplementation) on the total volume (ml 3 ) of food obtained by the first nestling in the brood (i.e., cowbird in a parasitized brood, randomly selected host chick in an unparasitized brood).................................................................................83 Figure 3. Mean (SE) total food (ml 3 ) obtained for first chicks in nests of the (A) sparrow, (B) blackbird, and (C) thrasher over two feeding treatments and relative to whether a brood contained a cowbird and a host chick or two host chicks................................................................................................84 Table 4. Relative proportion and total number (in parentheses) of identifiable prey types in parasitized and unparasitized broods of three host species ..................................................................................................................85 Figure 4. Mean (SE) latency to beg (s) per provisioning visit in (A) sparrow, (B) blackbird, and (C) thrasher broods that contained a cowbird and a host chick (parasitized) or two host chicks (unparasitized).....................86

xx Figure 5. Mean (SE) time spent begging (s) per visit in (A) sparrow, (B) blackbird, and (C) thrasher broods that contained a cowbird and a host chick (parasitized) or two host chicks (unparasitized).............................87 Figure 6. Mean (SE) maximum begging posture per visit in (A) sparrow, (B) blackbird, and (C) thrasher broods that contained a cowbird and a host chick (parasitized) or two host chicks (unparasitized).............................88

V. Figure 1. Mean (SE) latency to beg (s) for cowbird (filled circles) and blackbird chicks (open circles) over the course of experimental trials..114 Figure 2. Mean (SE) begging score for cowbird (filled circles) and blackbird chicks (open circles) over the course of experimental trials..................115 Figure 3. Mean (SE) latency to beg (s) for Kansas (filled circles) and Illinois cowbird chicks (open circles) over the course of experimental trials....116 Figure 4. Mean (SE) begging score for Kansas (filled circles) and Illinois cowbird chicks (open circles) over the course of experimental trials..... ................................................................................................................117

VI. Figure 1. Mean (SE) bill-equivalents of food delivered by vireos per 10 min to (A) single cowbird and single blackbird chicks and (B) broods of 1-4 vireo chicks............................................................................................141

xxi Figure 2. Mean (SE) number of feeding visits by vireos per 10 min to nests containing (A) single cowbird and single blackbird chicks or (B) broods of 1-4 vireo chicks..................................................................................142 Figure 3. (A) Mean (SE) time begging (s) per visit, (B) mean (SE) begging posture per visit, and (C) mean (SE) number of begging calls per visit for cowbird and blackbird chicks videotaped under field conditions..........143 Figure 4. Mean (SE) latency to beg (s) of cowbird (filled circles) and blackbird chicks (open circles) raised in vireo nests and tested under laboratory conditions...............................................................................................144 Figure 5. Mean (SE) begging score of cowbird (filled circles) and blackbird chicks (open circles) raised in vireo nests and tested under laboratory conditions...............................................................................................145

VII. Table 1. Parasitism rate (i.e., proportion of total nests parasitized), mean parasitism intensity (i.e., mean number of cowbird offspring per parasitized nest), and total Brown-headed Cowbird offspring found in host nests at Konza Prairie during the 2002-07 breeding seasons.........165 Figure 1. Selection indices and 95% confidence intervals for cowbird preference of the four primary host groups during 2006 (filled circles) and 2007 (open circles)..........................................................................166 Figure 2. Relationships between (A, B) relative selection indices and nest initiation date and (C, D) between the proportion of total cowbird

xxii offspring in each host group for each nest initiation period for the 2006- 07 breeding seasons................................................................................167 Figure 3. The percent of nests for each level of the number of cowbird eggs per nest during 2002-07 (filled circles) and 1965-79 (open circles), (B) relationship between parasitism rate (percent of nests parasitized), and (C) mean parasitism intensity (mean number of cowbird offspring per parasitized nests) relative to nest initiation date during 2002-07 (filled circles) and 1965-79 (open circles)........................................................168

1 I. General Introduction Begging, which can be described as the combination of vocalizations and physical movements to elicit resources from adults, is part of a dynamic interaction between dependent young and their care-providing parents (Kilner and Johnstone 1997, Budden and Wright 2001a). Dependent young use begging to signal their need for a critical resource (typically food) to parents; parents, in turn, use the signal given by offspring to make decisions as to how much of that resource to provide. This dynamic interaction between parents and their offspring has garnered much interest from evolutionary biologists for its relevance to signaling theory and because it appears that begging may be a way by which parent-offspring conflict may be resolved (Mock and Parker 1997, Godfray 1999). Numerous investigations of this interaction between parents and their offspring have focused on assessing the costs of begging, and two types of costs that appear to place an upper limit on the intensity of begging displays are now recognized: (1) direct costs such as increased predation risk, loss of energy, or both; and (2) indirect costs which decrease inclusive fitness through a reduction in the residual reproductive value of parents and/or the resources provisioned to broodmates that might otherwise receive the resources delivered to the begging individual (Mock and Parker 1997, Johnstone and Godfray 2002). Avian brood parasitism occurs when a female (the ‘brood parasite’) lays an egg into the nest of another individual (the ‘host’), and the host provides all parental care to the offspring of the brood parasite (Rothstein 1990, Davies 2000). Obligate brood parasitism is rare among birds and is limited to only ~1% of extant bird species in

2 seven unique lineages (Davies 2000, Sorenson and Payne 2005). These brood parasites are raised by host parents and, under most circumstances, are raised alone in the nest or compete against unrelated nestmates. Thus, begging by brood parasites is not typically constrained by indirect fitness costs, and young brood parasites are predicted to beg at a greater intensity for a given level of need than non-parasitic species (Harper 1986, Motro 1989, Holen et al. 2001). Indeed, early studies of brood parasitism often remarked at the exaggerated begging of parasite chicks (e.g., Friedmann 1929) and experimental work has confirmed this pattern for several species (e.g., Redondo 1993, Dearborn and Lichtenstein 2002). Nevertheless, begging behavior in brood parasites raises a number of important but unresolved issues, including such topics as how short-term need influences the intensity of the begging display, how parasitic chicks influence their host nestmates and parents via their begging behavior, and whether the begging display of a parasitic chick is more intense than a nonparasitic, closely related species when both species are raised in an identical nest environment. Clearly, much remains to be learned about brood parasites, and this dissertation is one attempt at expanding our knowledge of the behavioral ecology of interactions between a brood parasite, its foster parents, and the host offspring with which it interacts in the nest environment. In my dissertation, I conducted five experimental studies in two topical areas to better understand the factors that may influence begging behavior in a brood parasite that is an extreme generalist, the Brown-headed Cowbird ( Molothrus ater , hereafter cowbird). The first area of research is based on understanding how short-term need and nestmate size influence the begging intensity of cowbird chicks and the

3 provisioning they receive and, in turn, how cowbird begging influences the behavior of host parents and offspring. This work was motivated by the finding that that previous studies of begging in cowbirds and other generalist brood parasites were largely restricted to hosts with chicks that were smaller than the brood parasite for a given age (but see Lichtenstein 2001a for a notable exception). Given that larger chicks are more effective competitors for a given age, I set out in the first experiment (Chapter 2) to determine how cowbird begging intensity varied when competing against hosts of different sizes (smaller, similar to, and larger than cowbirds for a given age) while controlling for both short-term need and the number of nestmate competitors. At the same time, this study examined how much food cowbirds received when competing against hosts of different sizes to estimate the amount of food cowbirds gained by a given amount of begging behavior. I found that cowbird begging intensity was not influenced by short-term need, that nestmate size strongly influenced begging behavior, and that cowb irds obtained more food when competing against an intermediate-sized host due to the lower provisioning rates of small hosts or because of increased competitive ability of large host nestlings. This work was published as a single-authored paper in the journal Behavioral Ecology as a peer- reviewed manuscript in the January/February 2007 issue. The second experiment in my dissertation (Chapter 3) closely follows that of the first experiment and uses the same pool of chicks to document begging that occurred when parents were absent from nests. In particular, this work focused on understanding how cowbird and host chicks differed in the frequency of their “parent-absent begging” or in the types of stimuli they responded to during parent-absent periods. Like the first experiment, I

4 also examined how short-term need and nestmate size influenced the intensity of parent-absent begging by cowbird chicks. This study found that food-deprived cowbirds with hosts of different sizes begged more frequently and for a greater proportion of parent-absent periods than when food-supplemented. In contrast, three metrics of cowbird begging intensity varied relative to host size but not due to differences in short-term need. Cowbirds typically begged more frequently and intensively than host nestlings for a given level of short-term need, providing evidence that cowbird begging displays are more frequent and intense than non- parasitic nestlings during both feeding visits and parent-absent periods. This work was submitted as a single-authored, peer-reviewed manuscript to Behavioral Ecology and Sociobiology and at the time of this writing is in a revised version that is being assessed by their editorial office. The third experiment in my dissertation (Chapter 4) used the same experimental design as the two chapters, and its goal was to understand how the presence of a parasitic cowbird influenced the behavior of host adults and offspring. This was assessed by creating broods of two chicks (one cowbird and one host chick, or two host chicks) and quantifying whether parental provisioning and begging behavior of hosts ch anged with the presence of a cowbird chick in the nest and how this varied relative to short-term need and nestmate size. This study revealed that provisioning by host adults did not change in the presence of a cowbird but instead was influenced by feeding treatment and host size. The way in which begging intensity of cowbirds influenced host begging varied according to the size of the host: in the small host, the presence of a cowbird increased host begging intensity relative to unparasitized broods whereas in the large host, host

5 chicks begged less intensively with a cowbird present relative to when tested alongside a conspecific chick. This manuscript is co-authored with Tom Loughin and Steve Rothstein and will be submitted as a peer-reviewed contribution. The second area of focus for my dissertation centers on understanding whether the exaggerated begging of cowbird nestlings is an adaptation for brood parasitism. Although many studies have found that cowbirds have more intense begging than host nestmates, only a handful of studies have controlled for phylogeny. Furthermore, no study has examined begging by cowbirds and a close relative in a nest environment where both species were raised by heterospecific “foster parents.” Therefore, the fourth experiment of my dissertation (Chapter 5) is focused on determining if the exaggerated begging of the cowbird is shared with the nonparasitic, closely related Red-winged Blackbird ( Agelaius phoeniceus , hereafter blackbird) when both species are raised under identical nest conditions (in this case, alongside two host chicks in nests of the Dickcissel [ Spiza americana ]). Counter to predictions, cowbird chicks did not exhibit greater begging intensity than blackbird chicks for four distinct aspects of the begging display; indeed, blackbirds begged significantly more intensively than cowbirds for two of the begging measures. Based on these findings, this project was expanded to a second location (Illinois) to assess how the high relatedness of cowbird chicks on the original site in Kansas may have constrained begging intensity. Those additional experiments revealed that, when tested under identical conditions, Illinois cowbirds begged more intensively than Kansas birds for all four aspects of the begging display. A comparison of the two populations suggests that growth and nest predation costs did not differ between

Full document contains 221 pages
Abstract: Begging is a dynamic interaction in which dependent young solicit resources (typically food) from their care-giving parents, and parents use begging displays to make decisions about resource distribution among young. Theoretical studies indicate that the degree of relatedness between nestmates and the level of short-term need (i.e., hunger) experienced by an individual can strongly influence begging intensity. Obligate avian brood parasitism occurs when a female (the "brood parasite") lays an egg into the nest of another species (the "host"), which then provides all parental care to the parasitic chick. Because brood parasites are provisioned by unrelated "foster parents" and typically compete against unrelated nestmates, they are expected to act more selfishly and exhibit exaggerated begging behavior relative to nonparasitic species, all else being equal. This study was designed to elucidate the proximate and ultimate factors that may influence begging behavior in a generalist brood parasite, the Brown-headed Cowbird ( Molothrus ater ). The first three experiments assessed the extent to which two proximate factors (i.e., short-term need, nestmate size) influence cowbird begging, host begging, and provisioning by host parents. Taken together, these experiments found that short-term need generally increased the frequency and intensity of begging by cowbird and host chicks in the presence and absence of parents, that cowbirds typically begged more intensively than host chicks, and that a cowbird in the nest influenced host chick begging but not host food provisioning. Two additional experiments focused on the ultimate question of whether the exaggerated begging of the cowbird was an adaptation for brood parasitism and found that cowbirds did not exhibit begging displays that were more intense than those of the nonparasitic, closely related Red-winged Blackbird ( Agelaius phoeniceus ) as predicted by theory. Importantly, this relationship was consistent across two markedly different host species, and additional data suggest that high relatedness of cowbirds at Konza Prairie may constrain begging behavior. The final, observational study found that cowbirds parasitized nearly all available hosts at Konza Prairie, focused the majority of their parasitism on the Dickcissel ( Spiza americana ) and Bell's Vireo (Vireo bellii ), and were consistent in their use of hosts over the breeding season.