On fledging, the context in which bluebird offspring are fed changes dramatically. Young bluebirds leave the restrictive confines of the nest-box and spread out into the bright, open-field habitats typically occupied by the species. The change in both feeding environment and offspring proximity means that many of the signals used by bluebird parents to assess fledgling quality should be different than signals used to assess nestling quality. Mouth color, used by adults of many bird species to assess nestling need and quality (Gotmark and Ahlstrom 1997
; de Ayala et al. 2007
; Ewen et al. 2008
), is probably not as useful as signal outside the confines of the nest because fledged offspring are less likely to beg simultaneously when spatially separated (Ligon RA, personal observation), at least early in the fledgling stage. However, the flanges of many recently fledged passerine birds remain enlarged for a period of time after offspring have left the nest and these may serve as a signal for provisioning parents (e.g., de Ayala et al. 2007
; Dugas 2009
), both within and outside of the nest. Relative begging volume may also be more difficult for parents to assess after fledging, when nestlings are spatially separated (Mennill D, personal communication). Juvenile begging displays also attract predators (Haskell 1994
; Leech and Leonard 1997
) and may therefore carry greater costs outside of the nest. In the wild bluebirds that we watched, begging commenced only when parents approached fledglings, and this constrained behavior likely reflects the risks associated with extended periods of begging. By assessing condition-dependent plumage traits, parents are able to assess the relative quality of their offspring without forcing them to reveal themselves through overt begging displays.
Using feather brightness as a cue for assessing offspring quality makes sense because the brightness of blue primary feathers is a condition-dependent trait in juvenile bluebirds. Young male bluebirds raised in less crowded nests with access to more food grow brighter feathers (Siefferman and Hill 2007
). Prior studies with other bird species also found that juvenal plumage coloration can be a signal that alters the amount of food provided by parents. Direct (Tanner and Richner 2008
) and indirect (Galván et al. 2008
) measures of parental investment indicate that UV plumage coloration of great tit P. major
offspring influences the feeding decisions of parents. In both studies (Galván et al. 2008
; Tanner and Richner 2008
) UV blocking techniques were used that reduced UV reflectance of yellow juvenal plumage outside the range of natural variation. These studies provide important insights into the potential for juvenal plumage to serve a signaling function, but because they involve the virtual removal of UV reflectance, neither study shows how parents respond to natural variation in color expression (Hill 2006b
By presenting bluebird parents with pairs of their offspring that differed in feather coloration, we tested for preferential feeding based on plumage coloration. In simple comparisons of food delivery attempts to fledglings with natural feather coloration, we found no significant bias in either males or females to feed brighter offspring more. When we used marking pens to color the plumage of the paired fledglings, making one relatively brighter and one relatively darker, we again found no significant bias in the food delivery of parents related to offspring color. From these observations, we conclude that provisioning without regard to feather coloration is a common behavioral tactic in eastern bluebirds.
When we added more details to our analyses about the circumstances in which the feeding trials were performed, interesting patterns of provisioning related to feather coloration began to emerge. In the control period, females provisioned naturally brighter sons more when they were heavier than darker brothers, when trials occurred earlier in the breeding season and when fathers fed these sons more during the control period. During this control period, males preferentially fed brighter sons in relation to the degree that these sons were brighter than their brothers. Thus, while the simple paired comparisons, in which siblings were ranked simply as brighter or drabber regardless of the difference in feather coloration, showed no pattern of preferential feeding, when additional variables were considered it became apparent that females shifted to provisioning brighter sons more when circumstances favored greater allocation to superior offspring. Males did not show the same sort of subtle shifts in strategy but rather seemed to respond only to the magnitude of difference in ornamental display, which could be interpreted as responding to the magnitude of difference in the quality of offspring.
When considering multiple explanatory variables during the experimental period, we found that mothers increased provisioning attempts to relatively brightened offspring if they had higher UV chroma, if they were naturally brighter, if it was earlier in the season, if they were heavier, if fathers fed them more during the experimental period, and if fathers contributed relatively less to nestling provisioning. Hence, as in the control period, when certain conditions favored preferential food delivery to superior offspring, females shifted to greater allocation of food to brighter chicks. Paternal feeding rates during the experimental period were influenced by relatively few variables, with males responding only to brood size and differences in manipulated brightness between sons. The brighter that sons were relative to their brothers during this period, the more feeding attempts they received from their fathers. This relationship was strongest in fathers of large broods, likely because of the increased burden placed on fathers provisioning multiple offspring.
Although brood size and relative differences in the brightness of fledglings were the only variables influencing paternal feeding decisions, several factors influenced the relationship between maternal feeding decisions and fledgling plumage color. In both control and experimental periods, females increased feeding attempts to brighter or brightened sons early in the season, at a time when temperatures are consistently cooler. Lower daytime and nighttime temperatures might reduce the abundance of arthropod prey items necessary for self and nestling provisioning (e.g., Williams 1961
; Kamata and Igarashi 1995
) and increase thermoregulatory requirements of nestlings (Thessing 2000
). Nestlings that must invest proportionally higher energy in homeostasis would have higher nutritional requirements than nestlings raised without such constraints (Dawson et al. 2005
). Increased nutritional requirements of offspring could limit parental ability to adequately provide sufficient food for all nestlings, thus increasing the likelihood that parents might invest preferentially in those offspring most likely to survive, that is, those exhibiting the highest expression of a condition-dependent plumage trait. Additionally, increased thermoregulatory requirements of nestlings earlier in the season might also place undue pressure on brooding eastern bluebird mothers (eastern bluebird fathers do not brood; Gowaty and Plissner 1998
) contributing to the female-specific influence of date on feeding decisions.
Another variable that influenced maternal feeding decisions in both control and experimental periods was the relative mass of the fledglings. Females fed both naturally brighter and experimentally brightened sons more when they were heavier than their brothers. Juvenal plumage ornamentation and mass are probably not independent characters, given that previous research has shown juvenile males raised in enlarged broods with lower feeding rates have duller feathers (Siefferman and Hill 2007
). This relationship may explain some of the observed correlation between mass and maternal feeding preferences for naturally brighter offspring. However, the fact that experimentally brightened fledglings were also more likely to receive increased maternal investment if they were heavier than their siblings suggests that mothers can assess mass or mass-related character differences between fledglings. When analyzing offspring and making feeding decisions that optimize their own reproductive success, parents likely rely on a suite of cues to determine the relative quality and qualities of their offspring. Based on its influence on maternal feeding decisions in both control and experimental periods, the relative mass of fledglings appears to be one such cue.
Although differential fitness payoffs for mothers and fathers in different environments and contexts can lead to different investment strategies (e.g., Krebs and Magrath 2000
; Kilner 2002
; Quillfeldt et al. 2004
), both parents are generally better off when their offspring survive. A certain degree of correlation in feeding behaviors between mothers and fathers is therefore not unexpected and we found that eastern bluebird mothers were more likely to feed naturally brighter and experimentally brightened offspring if their mates fed these same offspring at higher rates. However, the fact that paternal investment was not related to maternal feeding decisions is interesting. Given that female investment strategies appear to incorporate several sources of information (e.g., date, mass, color) it would seem that males could benefit by incorporating maternal feeding strategies into their own optimal provisioning decisions. However, given that paternal investment during the fledgling stage is lower than that of mothers, the benefits of assessing multiple sources of information to optimize feeding strategies (as females appear to do) may be low, resulting in an optimal paternal strategy that incorporates only direct and easy-to-assess fledgling cues like plumage brightness and brood size.
The importance of reliable signals of offspring quality likely varies with resource availability and the costs of obtaining food. Such signals should be less important when resources are abundant and the costs of provisioning an entire brood are low. When resources are limited or the cost of obtaining resources is high, however, signals of offspring quality may be very valuable because they allow parents to maximize reproductive success by investing heavily in high-quality offspring. Habitat quality, time of year, and the relative investment of one's mate are just a few of the many factors could influence the costs of provisioning an entire brood. In our experiment, mothers increased the proportion of feeding attempts to experimentally brightened sons when the relative investment of fathers during the nestling stage was low. Fathers that provision nestlings at low rates increase the demands on mothers, thereby limiting the ability of mothers to provide adequate food for their entire brood. The increased demands on mothers mated to low investing fathers may make mothers more discriminating when delivering food items to offspring, in order to maximize their own reproductive success. Fathers with larger broods were also more likely to increase investment in experimentally brightened offspring suggesting that the increased costs of raising more offspring also influences the feeding rules and decisions of fathers. It would be interesting to measure additional factors such as food abundance and habitat quality to determine the impact of these variables on parental feeding decisions.
Natural selection and phylogenetic constraints have long been assumed to shape the appearance of juvenal plumage, but the juvenal plumages of some birds have characteristics of complex intraspecific signals of condition. Species such as the eastern bluebird, with well-understood, sexually selected signals of quality and condition, provide opportunities for exploring the potential intraspecific signaling functions of juvenile traits. Recent studies show that the coloration and pattern of juvenal plumage function in much more than camouflage. Future studies should focus on the types of signals used in parent–offspring communication, the environmental conditions that favor the use of such signals, and the mechanisms ensuring the honesty and stability of these traits.