Male brood provisioning rates provide evidence for inter‐age competition for mates in female Cooper's Hawks Accipiter cooperii

Abstract

Life history theory predicts that individuals should maximize lifetime reproductive success (LRS) by breeding as soon as they reach sexual maturity, yet many species delay breeding, either because there are insufficient available mates or breeding sites, or because delayed breeding yields higher LRS. Accipitriform species, such as Cooper's Hawk Accipiter cooperii, exhibit both delayed breeding and delayed plumage maturation. However, in certain circumstances, first‐year females in non‐definitive plumage do breed and apparently compete with older females for high‐quality breeding territories. We predicted that these young females are at a competitive disadvantage compared with older females and that older females would have both higher reproductive success and be able to acquire higher quality nesting territories. We conducted brood counts and measured prey delivery rates by male Cooper's Hawks in an expanding urban population located in Albuquerque, New Mexico (USA), to assess our prediction. We found that older females had higher reproductive success, fledging 1.6 more offspring than younger females, and that they occupied territories where males provisioned at higher rates of 0.37 more prey items per 2‐h period. Our results showed that older females fared better than first‐year females but it is unclear if this is the result of passive or active competition. Older females initiated nesting 14.3 days sooner than first‐year females and thus may have filled vacant, high‐quality territories before first‐year females began seeking mates. Additionally, first‐year females were never observed persistently to confront older females for breeding territories, but they did actively compete against each other. First‐year females may defer to older females who, in a direct competitive interaction, would be most likely to prevail. Thus, delayed plumage maturation in Cooper's Hawks may serve to focus competition for nesting territories within age classes.

A two‐sex integrated population model reveals intersexual differences in life history strategies in Cooper's hawks

“…In our study population, the frequency of territory occupancy by FY females varied greatly across years and was most strongly inversely correlated with rates of AFY female immigration. Older, experienced raptors are usually dominant over younger individuals in acquiring a breeding territory (Ferrer & Bisson, 2003; Lien et al, 2015; Newton, 1979), and consequently, the ebb and flow of FY female breeders was likely determined by the availability of AFY female immigrants. There was no relationship between the frequency of FY female breeders and survival of either AFY males or females or with fecundity, so the drivers of this relationship appeared independent of demographic conditions in our study area.…”

“…Many Cooper's hawks (Figure 1) initially captured and tagged with transmitters in the urban study area dispersed elsewhere in New Mexico and southwestern Texas, and we continued to monitor these hawks where they settled. For a more thorough description and maps of the area, see Lien et al (2015), Millsap (2018), and Millsap et al (2019).…”

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

“…We found that for both sexes, older kestrels began their breeding attempts earlier than those in their first attempt. Previous studies have also demonstrated a significant relationship between the date of clutch initiation and breeding success in a variety of species, including Eurasian Kestrels ( Falco tinnunculus ; Dijkstra et al 1990), Cooper's Hawks ( Accipiter cooperii ; Lien et al 2015), Eurasian Blackbirds ( Turdus merula ; Jankowiak and Wysocki 2016), and House Finches (Badyaev and Hill 2002). Clutch size, hatching rate, and overall breeding success have been observed to decline over the course of the breeding season past an optimal reproductive window for Snow Geese ( Anser caerulescens ; Bêty et al 2003), Western and Clark's Grebes ( Aechmophorus occidentalis and A. clarkii ; Robison et al 2015), Western Sandpipers (Ruthrauff 2002), Common Murre ( Uria aalge ; Wanless and Harris 2007), Roseate Terns (Burger et al 1996), and Song Sparrows (Nol and Smith 1987).…”

“…Intersexual selection in favor of more experienced breeding partners has been demonstrated in raptors such as Peregrine Falcons ( Falco peregrinus ; Zabala and Zubergoitia 2014), Eurasian Sparrowhawks ( Accipiter nisus ; Newton et al 1981), and Cooper's Hawks (Lien et al 2015, Millsap et al 2019), as well as other species including American Flamingos ( Phoenicopterus ruber ; Cezilly et al 2008), Great Tits ( Parus major ; Perrins and McCleery 1983), and House Finches (Badyaev and Hill 2002). Among American Kestrels, females presumably can assess the quality of potential mates using indirect cues such as the color intensity of the cere and legs, as well as the brightness of his plumage, indicating good diet and overall health (Wiehn 1997, Bostrom and Ritchison 2006, Smallwood and Bird 2020).…”

Causes of Death of Female Cooper's Hawks from an Urban Setting in New Mexico, USA

“…data); during fall some individuals migrated southward up to 1600 km, as far as central Mexico. For a more thorough description and maps of the area see Lien et al (2015), Millsap (2018), andMillsap et al (2019).…”