Climate change affects bird nesting phenology: Comparing contemporary field and historical museum nesting records.

Global climate change impacts species and ecosystems in potentially harmful ways. For migratory bird species, earlier spring warm-up could lead to a mismatch between nesting activities and food availability. CO2 provides a useful proxy for temperature and an environmental indicator of climate change when temperature data are not available for an entire time series. Our objectives were to (a) examine nesting phenology over time; (b) determine how nesting phenology relates to changes in atmospheric CO2 concentration; and (c) demonstrate the usefulness of historical museum collections combined with modern observations for trend analyses. We assessed changes in nesting dates of 72 bird species in the Upper Midwest of the United States by comparing contemporary lay dates with those obtained from archived, historical museum nest records over a 143-year period (1872-2015). Species-specific changes in lay date per one unit change in the CO2 residual ranged from -0.75 (95% CI: -1.57 to -0.10) to 0.45 (95% CI: -0.29 to 1.43). Overall, lay dates advanced ~10 days over the 143-year period. Resident, short-distance migrants and long-distance migrants lay dates advanced by ~15, 18 and 16 days on average respectively. Twenty-four species (33.3%) significantly advanced, one (1.4%) significantly delayed and we failed to detect an advance or delay in lay date for 47 species (65.3%). Overall mean advance in first lay date (for the species that have significantly advanced laying date) was 25.1 days (min: 10.7, max: 49.9). Our study highlights the scientific importance of both data gathering and archiving through time to understand phenological change. The detailed archived information reported by egg collectors provide the early data of our study. As with studies of egg-shell thinning and pesticide exposure, our use of these data illustrates another scientific utility of egg collections that these pioneer naturalists never imagined. As museums archive historical data, these locations are also ideal candidates to store contemporary field data as it is collected. Together, such information will provide the ability to track, understand and perhaps predict responses to human-driven environmental change.

Increased egg-nest visual contrast does not induce egg ejection in the eastern phoebe (Sayornis phoebe), an accepter host of the brood parasitic brown-headed cowbird (Molothrus ater).

Hosts of obligate brood parasitic birds can offset the costs of parasitism by rejecting foreign eggs. Like the vast majority of brown-headed cowbird (Molothrus ater) hosts, the eastern phoebe (Sayornis phoebe) paradoxically incurs substantial fitness costs by accepting cowbird parasitism. We investigated whether acceptance of brown-headed cowbird eggs may be mediated via lack of recognition due to crypsis in eastern phoebe nests, and whether egg-rejection behavior could be induced by manipulating egg-nest visual contrast as a means of facilitating parasitic egg detection in an artificial parasitism experiment. We found that brown-headed cowbird eggs have significantly lower achromatic, but not chromatic, visual contrast against the natural nest lining relative to phoebe eggs, suggesting that cowbird eggs may be cryptic in the low ambient light environments of natural or artificial covered ledges where eastern phoebes tend to nest. We also found that despite successfully increasing egg-nest visual contrast in our treatment, eastern phoebes did not reject model parasitic eggs. Our findings are consistent with previous work showing that increased egg-nest visual contrasts do not increase parasitic egg discriminability and rejection in hosts of avian brood parasites. Because sensory modeling showed that eastern phoebes were predicted to be able to visually perceive natural and experimental parasitism events, these hosts are suggested to lack the cognitive flexibility necessary to reject dissimilar eggs in their nests. Future work should determine whether this and other accepter species can be made to reject parasitism by using a more diverse array of artificial egg stimuli. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Molecular tracking of individual host use in the Shiny Cowbird - a generalist brood parasite.

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120-bp fragment of the mtDNA control region, we reconstructed full-sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk-browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.

Temporal patterns of host availability, brown-headed cowbird brood parasitism, and parasite egg mass.

I studied relationships between temporal patterns of host availability, brood parasitism, and egg mass for the parasitic brown-headed cowbird (Molothrus ater). At a study site consisting largely of edge habitat in north-eastern Illinois, I found 834 bird nests from 27 species. A total of 407 cowbird eggs and nestlings were found in these nests over three laying seasons. Nearly all (n= 379; 93%) were found in the nests of seven host species. For these species and all taken together, weekly nest availability generally decreased whereas parasitism frequency generally increased throughout the cowbird laying season, but the proportions of nests parasitized and the mean number of cowbird eggs in them did not. Additionally, no correlation was found between the proportions of nests parasitized and nest availability. Cowbird egg mass generally increased throughout the laying season, indicating that foraging conditions improved and that, early in the laying season, egg mass and quality may be less important than quantity. Consistently high weekly levels of parasitism indicate that cowbird reproduction was less limited by resources needed for egg production and more by the availability of suitable host nests. Fluctuating weekly host availabilities suggest that previously established, constant rates of cowbird egg laying would produce an excess of eggs during periods of low host availability. Further, the low frequency of parasitism (1%) of nests in stages too advanced for successful parasitism, and of abandoned nests, is consistent with the hypothesis that cowbirds' consistently high rate of egg production helps assure an egg is available when an appropriate nest is found. Frequently, nests were parasitized multiple times, raising the possibility that cowbirds were interfering with their own reproduction. A diverse host community increases the possibility that a decline of any one host species is unlikely to affect cowbird reproduction significantly.

Community-wide patterns of parasitism of a host "generalist" brood-parasitic cowbird.

The brown-headed cowbird (Molothrusater) is a generalist obligate brood parasite. Despite intensive study and growing concern over the negative impact of cowbird parasitism on populations of many hosts, very little is known about the factors influencing community-wide patterns of cowbird parasitism. Using systematic nest searches, nest parasitism was studied over two breeding seasons at a study site in northeastern Illinois encompassing grassland, forest-edge, and forest habitat, supporting a diverse avian community. Parasitism was observed for 18 out of 34 altricial bird species found nesting at the study site. A total of 299 cowbird eggs and nestlings were found in 191 of a total of 593 nests. Analyses revealed several ecological and behavioral factors associated with frequency of parasitism and the resulting distribution of cowbird eggs. Much higher frequencies of parasitism were found in edge and forest habitats than in grassland. Within the edge habitat, open nests were parasitized significantly more often than cavity nests. Among open nests in the edge habitat, the two largest species were never parasitized. Host behavior, particularly egg-ejection behavior, was associated with a reduced observed frequency of parasitism, but at least three species known to eject cowbird eggs were sometimes parasitized. For six common hosts capable of rearing cowbirds, we found no correlation between level of parasitism and host nest-survivorship, suggesting that fine-grained assessments of host quality by female cowbirds do not influence patterns of parasitism among acceptable host species, or that differences in host quality are not great and/or predictable enough for such fine-grained assessments. Our results suggest that when a variety of possible nests are available, the level of parasitism on a particular species is a balance between a␣cowbird's preference for a particular species and the effectiveness of host species' defenses. A conceptual model was developed that incorporates the observed correlation of cowbird eggs or nestlings with habitat, nest-type, host species' body mass, and host behavioral defenses. Additional community-wide studies of cowbird parasitism will test if this model is applicable to other avian communities.