Condition and Vulnerability to Hunting among Mallards Staging at Lake St. Clair, Ontario

Populations of migratory birds are usually considered to be limited by conditions in breeding or wintering areas, but some might be limited by conditions encountered on migration. This could occur at stopover sites where competition for restricted food supplies can reduce subsequent survival or breeding success, or during the flights themselves, when adverse weather can occasionally kill large numbers of individuals. Competition for food could act in a density-dependent manner and help to regulate populations, whereas weather effects are more likely to act in a density-independent manner. The evidence for these views is explored in this paper. When preparing for migration, birds must normally obtain more food per day than usual, in order to accumulate the body reserves that fuel their flights. Birds often concentrate in large numbers at particular stopover sites, where food can become scarce, thus affecting migratory performance. Rates of weight gain, departure weights, and stopover durations often correlate with food supplies at stopover sites, sometimes influencing the subsequent survival and reproductive success of individuals, which can in turn affect subsequent breeding numbers. Many studies have provided evidence for interference and depletion competition at stopover sites, relatively few for migration conditions influencing the subsequent breeding or survival of individuals, and even fewer for effects on subsequent breeding numbers. Migrants in flight occasionally suffer substantial mortality in storms, especially over water, sometimes involving many thousands of birds at a time. Other mass mortalities have resulted from atypical 'winter-like' weather, occurring soon after the arrival of summer migrants in their breeding areas or just before their departure in autumn. Again, many thousands of birds at a time have been killed in such incidents, causing reductions of 30-90% in local breeding densities. In some bird species, migration-related events can at times have substantial effects on the year-to-year changes in breeding population levels. Nonetheless, the difficulties involved in investigating migrating birds at different points on their migration routes have so far limited the number of studies on the influence of events during migration periods on population levels.

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“…Lightest birds were more likely to be shot in the ensuing months Dufour et al 1993 Lesser scaup Ayhtya affinis…”

Section: Body Condition and Subsequent Performancementioning

confidence: 99%

Can conditions experienced during migration limit the population levels of birds?

2006

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“…, Dufour et al . ). This suggests a condition bias in the harvest, which could lead to an over‐representation of males among the wing surveys if they were in poorer body condition.…”

mentioning

confidence: 97%

Could falling female sex ratios among first‐winter northwest European duck populations contribute to skewed adult sex ratios and overall population declines?

Fox

Cristensen

2018

Ibis

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“…However, this should not affect the results since the goal of the study was to calculate the rate at which individuals gain body mass rather than describing average body mass in the population. Using baited traps may also bias sampling towards food-stressed individuals (Dufour et al 1993). However, in this case one would predict that the body mass of birds recaptured at least once would be lower than that of birds never caught again after initial ringing, which was not the case (see below).…”

Section: Study Sitementioning

confidence: 97%

Fuelling rates of garganey (Anas querquedula) staging in the Camargue, southern France, during spring migration

et al. 2004

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Most species of long-distance migratory birds put on energy stores to fuel their travels. However, recent studies have highlighted the potential costs associated with carrying too much fuel, either through increased predation risk or decreased flight efficiency. Consequently, it is now widely accepted that migratory birds should carry optimal rather than maximum fuel loads. Information from 372 garganey (Anas querquedula) ringed and recaptured at least once during the same spring in the Camargue, southern France, was used to document fuelling rates of individual ducks in relation to environmental variation and individual variation in condition. On average, garganey added very little fuel stores in the Camargue (mean gain per day=0.33 g, less than 0.5% of mean body-mass in total over an average stay of 5 days). Fuelling rates were negatively correlated with body mass at capture, but it cannot be excluded that this pattern was a statistical artefact. Given their body-mass at ringing, garganey could potentially still fly long distances when they stop in the Camargue. It is therefore likely that the aim of their stay in southern France is more for resting than refuelling, a finding that may have implications for the proper management of stop-over sites.

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Condition and Vulnerability to Hunting among Mallards Staging at Lake St. Clair, Ontario

Cited by 69 publications

References 13 publications

Can conditions experienced during migration limit the population levels of birds?

Can conditions experienced during migration limit the population levels of birds?

Could falling female sex ratios among first‐winter northwest European duck populations contribute to skewed adult sex ratios and overall population declines?

Fuelling rates of garganey (Anas querquedula) staging in the Camargue, southern France, during spring migration

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