Exceptional variation in the appearance of Common Murre eggs reveals their potential as identity signals

We studied the ground colors and maculations of 161 Common Murre (Uria aalge) eggs laid by 43 females in 3 small breeding groups on the cliffs of Skomer Island, Wales, in 2016–2018. Both the colors and maculations varied much more among than within females, providing quantitative evidence for the egg traits that might facilitate the parents’ ability to identify their own eggs on the crowded breeding ledges where the density is typically ~20 eggs m–2. Ground colors had a trimodal distribution of hue values (whitish to pale brown, pale blue, or vivid blue-green) and maculations ranged from none to complex squiggles and blotches. The eggs laid by each female in different years were similar to one another, and replacement eggs laid by females within years were also more similar to their first egg than to other eggs in the same breeding group. Egg appearance did not differ among the 3 breeding groups that we studied. Our findings thus support anecdotal observations that, within and between years, female Common Murres lay eggs that have similar ground colors and maculations. We do not, however, find evidence that there is much difference among the eggs laid in different parts of a colony.

Inter-colony foraging dynamics and breeding success relate to prey availability in a pursuit-diving seabird

Spatial patterns of breeding seabirds are influenced by the distribution of resources in relation to the colony and the density of conspecifics from the same or adjacent colonies. We conducted an inter-colony comparison of foraging space use and behavior, diet, and reproductive success of common murres Uria aalge breeding at a large offshore and a small inshore colony on the northeastern coast of Newfoundland (Canada) during 2016-2018 under varying prey (capelin Mallotus villosus) biomass. Murres from the large offshore colony foraged over a greater area, with greater individual foraging distances, indicative of higher commuting costs compared to the smaller inshore colony. Although this pattern might reflect prey depletion near the offshore colony due to higher conspecific densities, it likely also reflects the greater distance to predictable, high-abundance prey aggregations. This is supported by high spatial overlap of foraging areas from both colonies near coastal, annually persistent capelin spawning sites. Adult diet was similar between colonies during incubation, but diverged during chick-rearing, with offshore murres consuming a higher proportion of alternative prey, while inshore murres consumed more capelin. These differences did not affect fledging success, although hatching success was lower in the larger colony, suggesting that divergent factors (e.g. predation, nest attendance) influence colony-specific population dynamics. Overall, our findings suggest that abundant local prey is key in shaping spatial patterns of breeding common murres in northeastern Newfoundland and results in apparently minimal intraspecific competition. As anthropogenic pressures on resource availability heighten, insight into factors influencing intraspecific foraging niche dynamics will be critical to inform management.

Interannual repeatability of eggshell phenotype in individual female Common Murres (Uriaaalge)

The recognition of own progeny is critical in group-living organisms that provide parental care for their young. The colonial seabird Common Murre (Uria aalge (Pontoppidan, 1763); also known as the Common Guillemot) does not build a nest, so direct cues must be available for the parents to recognize their own egg. However, only anecdotal evidence exists that, as seen in other avian lineages where examined, eggshells of Common Murres are also consistent in most aspects of their appearance between different breeding attempts by each female. Using digital photography, we quantified several eggshell traits of a handful of captive Common Murres across multiple years. Individual female Common Murre eggs were significantly repeatable in background colour, maculation coverage, spot shape, and shell size. Laying individually consistent and recognizable eggshells across breeding attempts may benefit Common Murres by reducing both the cost of relearning and the risk of misidentifying their own eggs. More generally, these data also add to the growing knowledge of individually consistent eggshell genesis by the avian reproductive system.