Seasonal trends in adult apparent survival and reproductive trade-offs reveal potential constraints to earlier nesting in a migratory bird

The American Kestrel (Falco sparverius), a small, cavity-nesting falcon that breeds across most of North America, is experiencing an apparent population decline across much of the United States that has yet to be sufficiently explained. With landscape change as a potential factor, the association of landscape composition with kestrel reproduction may assist not only in managing declines but also in improving the effectiveness and efficiency of nest box programs, which can bolster local populations and support monitoring and research efforts. We monitored 163 unique American Kestrel nest boxes over a 12-yr study period (2008–2019) in western Virginia to investigate the factors influencing kestrel reproductive success and we documented relatively high rates of both box occupancy (80%) and nest success (78.9%). In a study area dominated by both high- and low-intensity agriculture, we focused on identifying patterns in land cover composition that may limit reproductive success and associated each box with land cover composition metrics at four spatial scales. Generalized linear mixed models indicated nest boxes with a lower proportion of developed land within 500 m, and a higher proportion of crops and pasture/hay within 3 km were more likely to be occupied by kestrels. Although probability of kestrel nest success increased with reduced proportions of developed land within 100 m, the primary factors influencing success were clutch initiation date and the presence of European starlings (Sturnus vulgaris), with earlier nesting and the absence of starling activity resulting in higher nest success. Overall, cover-type associations with nest success and box occupancy were not strong and establishment of nest boxes in targeted kestrel habitat, while likely maximizing reproductive output, limits research opportunities to identify critical land cover thresholds for the species.

Section: Discussionmentioning

confidence: 99%

Factors Associated with American Kestrel (Falco sparverius) Nest Box Occupancy and Reproductive Success in an Agricultural Landscape

Kolowski

Morrow

2022

“…In our study area the relationship between laying date and kestrel ecology is complex, as described in a recent paper that compares the New Jersey population to a population in Idaho (Callery et al 2022). During recent decades spring has been advancing over both eastern and western regions of the USA.…”

Section: Discussionmentioning

confidence: 99%

“…Otherwise, clutch initiation date was unknown. Although this population had not experienced a significant change in clutch initiation date during recent years (Callery et al 2022), in order to account for slight interannual variation we calculated relative laying date by subtracting from each ordinal date of clutch initiation the mean ordinal date for that year. For analysis, relative laying dates <0 were considered early and relative laying dates >0 were considered late.…”

Section: Methodsmentioning

confidence: 99%

“…We tested the hypothesis that older kestrels were more prolific than those in their first reproductive attempt. This hypothesis is of particular interest because a recent study (Callery et al 2022) suggested that the Idaho and New Jersey populations differ with respect to reproductive trade-offs between the timing of breeding and the survival of adults and young; however, adult age was not included in that model. Here we examine for the first time the relationship of the age of breeding adults and reproductive performance in the New Jersey kestrel population.…”

Section: Introductionmentioning

confidence: 99%

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Reproductive Success Increases with Age in American Kestrels, Especially in Males

Snyder

Smallwood

2022

Self Cite

Age-related differences in reproductive or breeding success, as measured by clutch size, hatching rate, nestling survival, and number of fledglings produced, have been well documented in many species of birds. The experience gained through successive breeding attempts can improve foraging abilities, survival skills, access to resources through social dominance, and familiarity with each stage of reproduction. We examined 696 breeding attempts by American Kestrels (Falco sparverius) from a population that bred in nest boxes in northwestern New Jersey from 1995 to 2021. We tested the hypothesis that older kestrels have greater breeding success than those in their first breeding season. Clutch size, nestling survival rate, and the number of fledglings varied significantly and positively with the age of one or both parents. These trends were significantly correlated with the date of clutch initiation; older birds initiated clutches earlier than those in their first breeding attempt. However, after we controlled for laying date, older birds still had larger clutch sizes, higher nestling survival rates, and greater numbers of fledglings. The relationship between age and breeding success was more pronounced for males than females. This difference is consistent with the behavioral roles of each sex during a breeding attempt. After laying a clutch, females perform most of the incubation and brood young nestlings. In contrast, males provide food for females prior to egg laying and throughout incubation, and to the entire family until the nestlings no longer are brooded by the female. Females paired with older males had significantly greater body mass index values (weight/wing chord) than those paired with second-year males. Pairs where both adults were older had significantly greater breeding success than mixed or young pairs, and we never observed an older female paired to a male during his first breeding attempt.

“…Various hypotheses have been proposed for this decline. Some research indicates that climate change may be an underlying factor in many of the changes, as well as human-related modifications of habitat (Steenhof and Peterson 2009, Strasser and Heath 2013, Paprocki et al 2014, 2017, Smith et al 2017, Bossu et al 2022, Callery et al 2022a, 2022b). Many studies of American Kestrels have been conducted in the temperate regions of the continent (see Smallwood and Bird 2020 for an overview), but the causes of the declines are still largely unknown.…”

Section: Introductionmentioning

confidence: 99%

American Kestrel Nesting Biology and Long-Term Trends in the Alaskan Arctic: 2002–2021

Craig¹,

Craig²,

McMillan³

2023

We monitored a population of American Kestrels (Falco sparverius) nesting in boxes at the northern extent of the kestrel range (between 66° and 68°N) in the Alaskan Arctic, 2002–2021. There was no significant trend in occupancy during the study period but yearly variation in occupancy was high (range = 17–70%). Occupancy rate was positively related to the lowest temperature recorded in May (7–20°C). The mean estimated clutch initiation date was 16 May ± 6 d; we observed a slight but significant trend for later clutch initiation (4 d) during the study period. Kestrel clutch size averaged 4.7 ± 1.0 (range = 1–7), brood size averaged 4.6 ± 0.8, and the mean minimum number of young fledged/successful pair was 4.9 ± 0.4. Clutch and brood sizes remained stable from 2002–2021, with no significant trend. Nest failure was low (16%). We report a late nesting and possible double brooding attempt in 2018, suggesting a possible response to the warming trend (2002–2021) in average temperatures at the end of the normal nesting season.